Optimize .md exports for LLM agent page-relevance heuristics

[dcramer]

Per this observation from @faizan10114:

You still need to add index.md at the top of your markdown files. Purely for the purposes that most agents read the top 500 lines and decide if its the correct page or not (have data on this).

Most LLM agents read the first ~500 lines of a markdown file to decide whether the page is relevant before reading further. We should audit our .md exports to make sure the top of each page quickly conveys what it's about.

Things to investigate:

• Are titles and intro text descriptive enough in the first few lines?
• Should longer pages include a brief summary or TOC near the top?
• Are navigation sections (child page listings) pushing useful content too far down on index pages?
• Should index.md include a self-description to help agents identify it?
• Do we have data on which pages agents land on most and whether they bounce?

Related: PR #16417, specs/llm-friendly-docs.md

[linear]

DOCS-2428