docs: add the write-your-first-plugin walkthrough

[jouwdan]

Part of MEI-115 (deliverable 3 of 4: the authoring journey). Follows #258 (scaffolds) and #259 (smoke).

What this adds

docs/customization/first-plugin.md — the tutorial-shaped page the ticket asks for, taking an author from an empty directory to a listed plugin:

scaffold → change a hook → test it → install it into a board → publish → submit to the marketplace.

The existing customization pages are policy and reference: plugins.md says what a plugin may and may not do, and plugin-hooks.md lists every hook. Neither shows the path through them, which is the blank page a third-party author actually faces. This page assumes both exist and links out rather than restating them.

Registered in apps/web/content/docs.manifest.json and linked from docs/README.md (both the task table and the customization list); pnpm site:docs regenerated the root README table.

Two details worth flagging for review:

• The walkthrough's "Run it inside a board" step depends on test(extensions): prove scaffolded plugins and themes build against packed kits #259, which is what makes a board build compile an installed source-shipping extension. Merging this ahead of test(extensions): prove scaffolded plugins and themes build against packed kits #259 would publish a step that does not yet work.
• The test example deliberately ends on the don't-mutate-the-model assertion, since that is the mistake a first filter makes and the one the chain punishes silently.

Validated

pnpm site:docs:check, pnpm docs:index:check and pnpm docs:links:check all pass (43 documents, 733 headings, every internal link and anchor resolves).

🤖 Generated with Claude Code

https://claude.ai/code/session_01G22SRRgkUV7doefmw9NMNf

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