(AI) Add llm page hints

[hestonhoffman]

What does this PR do? What is the motivation?

We publish .md versions of most pages on the docs site to help LLMs read our documentation, but agents seem to have trouble finding these pages when a user asks an Agent to fetch and read a page.

This PR adds two methods to help the agent find the correct version of the page (if one exists).

If a .md version of the page exists:

• it adds an "invisible" div to the top of the page with directions to the .md version.
• It adds a meta entry to the page with the same link. This is a kind of fallback because some agents may automatically skip divs that are set to display:none. We haven't really landed on an industry standard for how agents search pages, so we have to cover a variety of bases.

How it works

1. Once per build, pages-json-cache.html uses partialCached to download pages.json from the docs site and store the data in a cached Hugo dict.
2. Once per page, page-agent-hint.html checks if a key exists in the cached dict and returns the .md URL if one exists. This also uses partialCached to prevent multiple calls for the same page.
3. If a key exists, the meta tag and div are added to the page.

Note:

• I borrowed a lot of this functionality from something @jhgilbert was working on.
• There is a bit of futzing with URL strings to get the correct .md path to the page.
• I needed to add this higher in the body element to get it to work