Add markdown download button and make llms.txt available

[neon60]

Motivation

AI coding assistants and agents increasingly need documentation in machine-readable formats. Two patterns are emerging as standards: per-page Markdown download (so users and agents can copy page content directly into an AI context window) and llms.txt (a combined documentation file that AI agents can discover and ingest). This PR adds both capabilities to make the documentation more accessible to AI-assisted workflows.

Technical Details

Two changes to docs/conf.py:

Enable the theme download button

Sets "use_download_button": True in html_theme_options, which activates the built-in per-page download action provided by rocm_docs_theme. This allows readers to download the current page as Markdown or RST depending on the source format.

Add llms.txt generation

Adds a Sphinx build-finished hook (generate_combined_markdown) that concatenates all Markdown source files into a single llms.txt at the output root. This follows the https://docs.readme.com/main/docs/LLMstxt for AI agent documentation discovery. Files in build, static, template, and VCS directories are excluded. The output is also compatible with the https://docs.readthedocs.com/platform/stable/reference/llms-txt.html.

Note: RST source files are not included in llms.txt at this stage.

Test Plan

• Verify the download button appears on documentation pages
• Verify llms.txt available for the project.
• Confirm excluded directories (.git, _build, _static, _templates, .venv) are not represented in llms.txt

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.