Add autogeneration of llms.txt during mkdocs build; add docs page on using llms.txt

[koverholt]

Automates generation of llms.txt in the docs root and adds a docs page for consuming it with AI coding tools. This PR:

• Adds support for llms.txt auto generation using the mkdocs-llmstxt package (updated requirements.txt and mkdocs.yml)
• Adds a documentation page for using llms.txt (with Gemini CLI, Antigravity, and general info for other clients)
• Remove manually generated llms.txt and llms-full.txt from Git (these will now be autogenerated and served at the docs site route)
• Fixed a bunch of docs warnings and INFO logs

After this PR is merged, the llms.txt will be autogenerated and available at:

• https://google.github.io/adk-docs/llms.txt
• https://google.github.io/adk-docs/llms-full.txt

And .md files will be automatically generated alongside the built HTML pages for easy ingestion by LLMs / coding tools.

Related issues:

• Provide a llms.txt file #68
• Provide language-specific versions of llms-full.txt #393
• Clarification on how llms.txt and llms-full.txt are generated #649
• Update llms.txt to align with the llms.txt standard and act as a sitemap for models #726

[koverholt]

The doc build CI check is failing due to --strict mode because mkdocs-llmstxt is emitting warnings related to unreferenced assets (images, JS, CSS). But the actual docs build will still pass. I'll see if I can figure out a good compromise for that failing test in this PR, otherwise I can handle it in a followup PR.

[koverholt]

I fixed the CI docs build warnings by reworking the globs in the mkdocs-llmstxt config in 0d09c7e and 9e39aee.

[koverholt]

I also opened an issue here: derailed-dash/adk-docs-ext#1 so that the Gemini CLI extension for ADK docs listed on https://geminicli.com/extensions/ will continue to work given the change to llms.txt in this PR.

[joefernandez]

Tested locally and reviewed. Thanks!