docs(website): give JSON its own API page

[unional]

Summary

• New api/json.md ("JSON"), holding the JsonTypes, JsonPrimitive, JsonObject and JsonArray section from type-sets.md: the JSON-boundary rationale, the details on undefined and optional properties, and the deprecated JSON* names, which link to the v8 migration guide. The page has an intro, a reference table and a Source link to src/json.ts. It is the last page in the sidebar (order 19), after Type Sets, so sidebar.order runs 1–19 with no gaps or duplicates and no other page is renumbered.
• required / requiredDeep moved to the Object page, after "Combining records". Their rows are in its runtime-functions table and its Sources line now includes src/unpartial.ts.
• type-sets.md is now "Type Sets". Its intro, description, reference table and Source list cover only PrimitiveTypes, ComposableTypes/NonComposableTypes and UnionKeys, and it links to JSON.
• generate-llms-txt.mjs: each file directly under src/ is now its own family, keyed by file name, in place of the single root family. This lets json.ts, unpartial.ts and the type-set files point at different pages. Both llms.txt files are regenerated.
• The package readme.md JSON section links to the new page, as docs(website): give Bigint its own API page #819 did for Bigint.

No link on the site targeted the moved sections, and neither did the migration guide. object.md's type-sets/#unionkeys link still works.

No changeset: like #819, this changes only docs, the readme and llms.txt.

Verification

• pnpm turbo run build --filter=website...: 0 errors. In the built HTML, api/type-sets/#unionkeys, guides/migrating-to-v8/#deprecated-in-80, and the JSON ↔ Type Sets links all lead to anchors that exist. The JSON page has no duplicate heading IDs.
• pnpm verify: passes, 7/7 tasks.

🤖 Generated with Claude Code

[changeset-bot]

⚠️ No Changeset found

Latest commit: bb62ad6

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9b2b94c) to head (bb62ad6).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           40        40           
  Lines          173       173           
  Branches        30        30           
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  Hits           173       173           

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