[SDK-10] Plugin-author documentation page

[ChrisonSimtian]

Author the plugin-author docs (walkthrough + API ref).

Status

Blocked on #41 (docs site migration) — once the docs site lands, write the page there. Until then, draft in a holding folder at docs/_drafts/plugin-author-guide.md so the prose is ready to drop in.

Scope

• Walkthrough: write a 'hello-world' plugin against the SDK, register one middleware, run it against a sample build.
• Reference: enumerate all six extension points (SDK-6a..6f), each with a code sample.
• Conflict resolution behaviour (SDK-4).
• Versioning policy explainer (SDK-5).
• Cross-link to the sample plugin (SDK-8) and testing helpers (SDK-9).

Depends on

#41 (unblocks the publish), SDK-1..SDK-9 (subject matter).