docs: split docs into per-project ArkType and Attest sections

[ssalbdivad]

Adds a project switcher to the docs sidebar using fumadocs root folders, starting with ArkType and Attest.

• Existing pages move into the content/docs/(arktype)/ folder group. Fumadocs strips groups from slugs, so every existing /docs/... URL is unchanged.
• New Attest docs at /docs/attest: setup, assertions, benches, options, CLI and integration. Examples are type-checked by twoslash. The README's stale option defaults and broken custom assertion example are fixed in the move.
• ark/attest/README.md keeps the intro and install and links to the docs instead of duplicating them.
• InstallationTabs accepts pkg/dev props.
• llms.txt is generated per section: /llms.txt covers ArkType (now including the cheat sheet), and /docs/attest/llms.txt covers Attest.

More packages can be added as sibling root folders (content/docs/<pkg>/meta.json with "root": true).

🤖 Generated with Claude Code

[pullfrog]

✅ No new issues found.

Reviewed changes

Reviewed the per-project docs restructure (60 files): existing pages moved into content/docs/(arktype)/, plus the new Attest docs section.

• Route-group move — pages relocated under (arktype) with "root": true. Verified against the installed fumadocs-core: getSlugs filters names matching /^\(.+\)$/, so every existing /docs/... URL is preserved, and DocsLayout derives the root toggle automatically from the tree.
• New Attest docs — six pages under content/docs/attest. Cross-checked the substantive examples against source: option defaults match getDefaultAttestConfig (ark/attest/config.ts), the .json snapshot matches divisor.test.ts, the error messages match structure/index.ts and divisor.ts, and the rewritten getTypeAssertionsAtPosition example matches the VersionedTypeAssertion/TypeRelationshipAssertionData shapes.
• InstallationTabs — new pkg/dev props produce correct install commands for pnpm/npm/yarn/bun; the existing <InstallationTabs /> call site still renders the same output.
• writeLlmsTxt — scoped to (arktype); regenerated public/llms.txt adds only the cheat sheet, consistent with the stated intent.
• Attest README trim + @ark/attest docs dependency — pnpm install --frozen-lockfile validates cleanly and prettier/eslint pass on the changed files.

No blocking or actionable issues found.

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