feat: improve AcceptMarkdown support

[dcrawbuck]

This adds AcceptMarkdown-compliant content negotiation for docs routes, including Fumadocs-based q-value negotiation, Vary: Accept, alternate/canonical Link headers, and 406 responses for unsupported media types.
It adds Markdown alternate URL helpers plus root and nested /llms.mdx/docs routes so /docs/, .md, .mdx, and canonical docs URLs all resolve to Markdown where appropriate.
It switches processed Markdown generation to Fumadocs placeholder rendering for common MDX components, making Markdown cleaner for agents while keeping HTML pages unchanged.
It adds tests for negotiation and rewrite behavior; local validation covered bun test, bun run build:cf, curl checks against local dev, and agent-browser checks.

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Note

Medium Risk
Changes global request middleware and URL routing for all docs traffic; behavior is well-tested but mis-negotiation could affect crawlers and LLM clients.

Overview
Adds AcceptMarkdown-style docs serving: Accept negotiation (including q-values and legacy markdown types), Vary: Accept, HTML Link alternates to markdown, and 406 for unsupported explicit types. Canonical /docs/..., .md, and .mdx URLs rewrite to markdown handlers when appropriate.

Introduces markdown-alternate URL/header helpers and nests /llms.mdx/docs routes (index + per-page) with canonical HTML Link headers; the docs index and page routes expose rel="alternate" markdown links and point copy/view actions at the new paths.

Processed markdown now uses Fumadocs mdxAsPlaceholder plus runtime renderers for common MDX components (tabs, callouts, etc.) so agent-facing output is cleaner; HTML pages are unchanged.

Tests cover negotiation, rewrites, and legacy redirects.

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[linear]

SW-5396

[cloudflare-workers-and-pages]

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	superwall-docs-staging	e202a5f	Commit Preview URL Branch Preview URL	Jun 03 2026, 11:52 PM