docs: add llms.txt for AI agents documentation discovery

[MiloCasagrande]

Add (a curated) llms.txt.

List of references should be manually or AI-assisted edited.
The file has been added under html_extra_path.

Signed-off-by: Milo Casagrande milo@foundries.io

PR Template and Checklist

Please complete as much as possible to speed up the reviewing process.
You may delete items that are not relevant to your contribution.
Readiness and adding reviewers as appropriate is required.

All PRs should be reviewed by a technical writer/documentation team and a peer.
If effecting customers—which is a majority of content changes—a member of Customer Success must also review.

Readiness

• Merge (pending reviews)
• Merge after date or event
• Draft

Checklist

• Run spelling and grammar check, preferably with linter.
• Step through instructions (or ask someone to do so).
• Review for wordiness
• Match tone and style of page/section.
• Run make linkcheck, and add redirects for any moved or deleted pages.
• View HTML in a browser to check rendering.
• Use semantic newlines.
• follow best practices for commits.
  • Descriptive title written in the imperative.
  • Include brief overview of QA steps taken.
  • Mention any related issues numbers.
  • End message with sign off/DCO line (-s, --signoff).
  • Sign commit with your gpg key (-S, --gpg-sign).
  • Squash commits if needed.
• Request PR review by a technical writer and at least one peer.

Comments

Any thing else that a maintainer/reviewer should know.
This could include potential issues, rational for approach, etc.