docs: update documentation links and images

[Ki-Seki]

Description

Summary: Correct docs links; use direct URLs to externally hosted images.

Reviewer: @J1awei-Yang

Checklist:

• I have performed a self-review of my own code | 我已自行检查了自己的代码
• I have commented my code in hard-to-understand areas | 我已在难以理解的地方对代码进行了注释
• I have added tests that prove my fix is effective or that my feature works | 我已添加测试以证明我的修复有效或功能正常
• I have added necessary documentation (if applicable) | 我已添加必要的文档（如果适用）
• I have linked the issue to this PR (if applicable) | 我已将 issue 链接到此 PR（如果适用）
• I have mentioned the person who will review this PR | 我已提及将审查此 PR 的人

[Copilot]

Pull Request Overview

The PR updates documentation links and switches image hosting from local assets to externally hosted URLs, ensuring all references point to the new memos-docs domain.

• Moved banner, logo, and other images to external CDN URLs
• Updated documentation and contribution links to the new memos-docs.openmem.net domain
• Adjusted markdown list items to plain URLs in the README

Reviewed Changes

Copilot reviewed 2 out of 7 changed files in this pull request and generated 1 comment.

File	Description
README.md	Replaced local image paths with external URLs; updated doc links; converted HTML links to plain URLs
.github/CONTRIBUTING	Updated contribution guide URLs to point to the new memos-docs domain

Comments suppressed due to low confidence (1)

README.md:47

• [nitpick] These list items were converted from <a> tags to plain URLs. While GitHub auto-links URLs, using standard markdown links [label](url) would improve readability and accessibility.

- **Website**: https://memos.openmem.net/