docs: dynamically generate llms.txt and llms-full.txt

[luarss]

Summary

Add support for llms.txt and llms-full.txt to the ReadTheDocs documentation site using the sphinx-llm extension. These files follow the llmstxt.org standard and make OpenROAD's documentation directly consumable by LLMs without HTML scraping.

Changes:

• Add sphinx_llm.txt to the Sphinx extensions list in docs/conf.py — the extension generates both files as a native build step, writing directly into the HTML output
• Add sphinx-llm to docs/requirements.txt
• bump python version 3.7 -> 3.11

Once merged and built, files will be served at:

• https://openroad.readthedocs.io/llms.txt
• https://openroad.readthedocs.io/llms-full.txt

Type of Change

• Documentation update

Impact

No change to tool behavior. Adds two new URL endpoints to the ReadTheDocs site after the next documentation build.

Verification

• I have verified that the local build succeeds (./etc/Build.sh).
• I have run the relevant tests and they pass.
• My code follows the repository's formatting guidelines.
• I have signed my commits (DCO).

Related Issues

N/A

[gemini-code-assist]

Code Review

This pull request introduces automated generation of llms.txt and llms-full.txt files to facilitate LLM documentation discovery. It adds a new Python script generate_llms_txt.py that parses the documentation structure from toc.yml to generate these files, integrates this script into the Sphinx configuration, and adds pyyaml to the documentation requirements. The review feedback suggests replacing legacy os.popen with subprocess.run to properly handle execution failures, explicitly specifying encoding="utf-8" for all file read and write operations to ensure cross-platform compatibility, and adding robust fallback handling for empty or malformed YAML parsing.

[luarss]

See
https://openroad--10598.org.readthedocs.build/en/10598/llms.txt
https://openroad--10598.org.readthedocs.build/en/10598/llms-full.txt