docs: add sitemap and middleware for serving Accept: text/markdown header

[jessiemongeon1]

Description

2 fixes to make the following agent-friendly docs tests pass:

FAIL: content-negotiation

Server ignores Accept: text/markdown header — 0/82 pages return markdown when requested via content negotiation. All return
text/html; charset=utf-8.

SKIP: llms-txt-coverage
No sitemap found; cannot assess llms.txt coverage

Test plan

Run tests against Vercel preview

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Project	Deployment	Actions	Updated (UTC)
sui-typescript-docs	Ready	Preview, Comment	Apr 30, 2026 5:00pm

[github-actions]

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