[docs] Add a CSS Modules to StyleX migration playbook

[ahmedsadid]

What changed / motivation ?

• Adds a conservative CSS Modules-to-StyleX migration playbook for LLM agents.
• Covers inventory, safe conversion, blocker reason codes, declaration conservation, and verification.
• Embeds the guide on the LLM Resources page using the existing scrollable copy interface.

Linked PR/Issues

Fixes #1810

Validation

Read every declaration in 40 CSS Modules files from two pinned OSS repos and recorded which can be safely converted. The rest must stay in CSS: their rendering depends on something the component doesn't own — an externally targeted class name, a DOM position, a consumer-overridable custom property — so converting them silently changes the page. We then ran the playbook once per file with gpt-5.6-sol.

Repository	Declarations	Safe to convert	Converted
Docusaurus	269	68	64
Umami	383	325	325
Total	652	393	389 (99%)

Zero unsafe conversions — nothing that should have stayed in CSS was converted. All 652 declarations accounted for.

Convertibility is a property of the codebase, not the playbook: Docusaurus is a published theme whose class names are a public API (25% convertible), Umami a self-contained app (85%).

Additional Context

• Canva discussion #1342.

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

[vercel]

@ahmedsadid is attempting to deploy a commit to the Meta Open Source Team on Vercel.

A member of the Team first needs to authorize it.

[nowshin-108]

LGTM! I'll let @mellyeliu weigh in.