docs: revise welcome page into overview and path selector

[lizongyu0912]

Summary

• Reworked the docs landing page into an Overview and Choose Your Path entry point.
• Added concrete AReno positioning, supported environment boundaries, and related-tool guidance.
• Added an Example Gallery page that routes users to quickstart, RLVR, and agentic RL examples.
• Updated the Get Started navigation from Welcome to Overview and linked the new gallery.

Validation

• LC_ALL=en_US.UTF-8 LANG=en_US.UTF-8 ../.venv-docs312/bin/sphinx-build -b html . ../dist/docs

Closes #107

[gemini-code-assist]

Code Review

This pull request restructures the documentation by updating the welcome page into a comprehensive overview and adding a new example gallery page. The overview page now details AReno's features, environment requirements, target audience, comparisons to other tools, and a path selection guide. Feedback on the changes points out a broken documentation link to the inference CLI, which would cause a Sphinx build error, and suggests correcting it to the proper reference path.

Important

The consumer version of Gemini Code Assist on GitHub is being sunset. Starting June 18, 2026, new organization installations will be blocked, and all code review activity will officially cease on July 17, 2026.
For more details on the timeline and next steps, please review the Help Documentation.

[gemini-code-assist]

The document path ../cli/inference does not exist in the repository. According to docs/index.rst, the CLI reference is located at reference/cli. Using ../cli/inference will cause a Sphinx build warning/error.

Please update this to point to ../reference/cli instead. Additionally, please note that there are raw HTML links to ../cli/inference.html and ../cli/training.html around lines 146 and 151 that should also be updated to ../reference/cli.html.

Suggested change

	- :doc:`Inference CLI <../cli/inference>`
	- :doc:`Inference CLI <../reference/cli>`