Improves contribution guidelines for IsaacLab

[Mayankm96]

Description

I realized there were some comments I often have to repeat in my reviewing process. I tried to add some of them into the code style page to directly point developers to it.

Type of change

• This change requires a documentation update

Checklist

• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there

[Copilot]

Pull Request Overview

This PR improves the contribution guidelines for IsaacLab by expanding documentation on code style, testing practices, and development workflows to help developers follow consistent patterns.

• Adds comprehensive code structure guidelines with examples showing proper organization of Python files and classes
• Expands unit testing documentation with platform-specific command examples for running tests
• Enhances the pull request template with clearer guidance on PR size and type descriptions

Reviewed Changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/source/refs/snippets/code_skeleton.py	New example file demonstrating proper Python code structure and organization patterns
docs/source/refs/contributing.rst	Major expansion of coding style guidelines, test running instructions, and code organization standards
docs/source/overview/environments.rst	Cleanup of duplicate image and link references
docs/conf.py	Minor documentation configuration updates
.github/PULL_REQUEST_TEMPLATE.md	Improved PR template with clearer guidance and descriptions

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