Add description metadata to first-level index pages (#3395)

## Summary

This PR adds `description` metadata to the frontmatter of all 10
first-level index.md files in the repository to improve SEO and provide
clear page summaries.

## Changes

Added concise, informative descriptions to the following first-level
index pages:

- **cloud-account/index.md** - Description about managing Elastic Cloud
account settings and preferences
- **deploy-manage/index.md** - Description about deploying and managing
Elastic environments
- **explore-analyze/index.md** - Description about exploring and
analyzing data with Kibana
- **extend/index.md** - Description about extending the Elastic Platform
(also added frontmatter structure)
- **get-started/index.md** - Description about Elastic fundamentals and
getting started
- **manage-data/index.md** - Description about data ingestion and
management in Elasticsearch
- **reference/index.md** - Description about Elastic Stack reference
documentation
- **release-notes/index.md** - Description about release notes and
changelogs (also added frontmatter structure)
- **solutions/index.md** - Description about Elastic solutions for
search, observability, and security
- **troubleshoot/index.md** - Description about troubleshooting
resources and support

## Implementation Details

- All descriptions follow the YAML multi-line format consistent with the
root `index.md` file
- Descriptions are concise (2-3 sentences) and accurately reflect the
content of each page
- Two files (`extend/index.md` and `release-notes/index.md`) previously
had no frontmatter and now have proper YAML frontmatter blocks
- All YAML syntax has been validated

## Related Issue

Addresses the requirement outlined in
elastic/docs-builder#1559

## Example

Before:
```yaml
---
applies_to:
  stack: ga
  serverless: ga
---
```

After:
```yaml
---
applies_to:
  stack: ga
  serverless: ga
description: Learn how to ingest, store, and manage data in Elasticsearch. Understand
  indices, mappings, text analysis, data lifecycle management, and migration between
  clusters.
---
```

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Add description metadata to first-level index
pages</issue_title>
> <issue_description>Most Markdown docs in this repository still lack a
`description:` in the frontmatter. We should at least add a description
for the first level pages, that is, the index.md pages inside each of
the docs folders.
> 
> Related issue:
https://github.com/elastic/docs-builder/issues/1559</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>
Fixes #3394

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: theletterf <[email protected]>

Author: Copilot

cloud-account/index.md

@@ -9,6 +9,9 @@ applies_to:
 products:
   - id: cloud-hosted
   - id: cloud-serverless
+description: Manage your Elastic Cloud account settings, including email, password,
+  authentication methods, and personal preferences. Configure your user profile
+  and customize your interface.
 ---
 
 # Manage your Cloud account and preferences

deploy-manage/index.md

@@ -9,6 +9,9 @@ products:
   - id: kibana
   - id: cloud-hosted
   - id: cloud-enterprise
+description: Deploy and manage your Elastic environment. Learn how to design resilient
+  clusters, secure access, monitor performance, and maintain your Elastic Stack
+  components across different deployment options.
 ---
 
 # Deploy and manage

explore-analyze/index.md

@@ -11,6 +11,9 @@ applies_to:
 products:
   - id: cloud-serverless
   - id: kibana
+description: Explore and analyze your data with Kibana. Query, filter, visualize,
+  and create interactive dashboards to gain insights from your Elasticsearch data.
+  Utilize machine learning, geospatial analysis, and more.
 ---
 
 # Explore and analyze data with Kibana

extend/index.md

@@ -1,3 +1,9 @@
+---
+description: Extend the Elastic Platform by creating integrations for data ingestion
+  or building custom applications with REST APIs. Learn how to contribute to Elastic
+  documentation and ecosystem.
+---
+
 # Extend and contribute 
 
 You can extend and build on the Elastic Platform in several ways. Whether you need to get data into Elasticsearch, add new functionality, edit the documentation, or build a custom application, here is an overview of the primary development paths to help you choose the right one for your project.

get-started/index.md

@@ -12,6 +12,9 @@ products:
 applies_to:
   serverless:
   stack:
+description: Learn the fundamentals of Elastic. Discover what Elastic offers, explore
+  core concepts of the Elastic Stack, understand deployment options, and access
+  training resources to get started.
 ---
 
 # Elastic fundamentals

manage-data/index.md

@@ -2,6 +2,9 @@
 applies_to:
   stack: ga
   serverless: ga
+description: Learn how to ingest, store, and manage data in Elasticsearch. Understand
+  indices, mappings, text analysis, data lifecycle management, and data migration between
+  clusters.
 ---
 
 # Manage data

reference/index.md

@@ -3,6 +3,9 @@ mapped_pages:
   - https://www.elastic.co/guide/en/starting-with-the-elasticsearch-platform-and-its-solutions/current/api-reference.html
 products:
   - id: elastic-stack
+description: Explore reference documentation for the Elastic Stack including APIs,
+  security schemas, configuration settings, query languages, scripting, ECS field
+  references, and more.
 ---
 
 # Reference [api-reference]

release-notes/index.md

@@ -1 +1,7 @@
+---
+description: Release notes and changelogs for Elastic products including Elasticsearch,
+  Kibana, Observability, Security, APM agents, and clients. Stay updated with new
+  features and changes.
+---
+
 # Release notes

solutions/index.md

@@ -6,6 +6,9 @@ applies_to:
   serverless: ga
 products:
   - id: cloud-hosted
+description: Learn about Elastic solutions for search, observability, and security
+  use cases. Get started with ready-to-use implementations and discover how to build
+  custom applications.
 ---
 
 # Solutions and use cases

troubleshoot/index.md

@@ -10,6 +10,9 @@ products:
   - id: elastic-stack
   - id: cloud-hosted
   - id: cloud-enterprise
+description: Troubleshooting resources and guidance for Elastic products including
+  Elasticsearch, Kibana, Observability, Security, and deployment platforms. Find
+  solutions and get support.
 ---
 
 # Troubleshooting