feat: add docs link to top bar and get-started modal

[FrkAk]

Summary

Task Reference: none — not tied to a tracked Piyaz task (UI follow-up to the docs.piyaz.ai launch).

Wires the app to the live documentation site (docs.piyaz.ai) in two places:

• TopBar — adds a persistent Docs link in the right action cluster, between Jump and the theme toggle. Styled to match the existing Jump button (same height/radius/tokens, label hidden below sm:, md: dividers on both sides). Opens https://docs.piyaz.ai in a new tab (target="_blank" rel="noreferrer", the app's existing external-link convention).
• Get Started modal — repoints the setup links from GitHub README anchors to the docs site, preserving the hosted/self-host split:
  • hosted → https://docs.piyaz.ai/docs/get-started/install
  • self-host → https://docs.piyaz.ai/docs/guides/self-host
  • Renamed getReadmeSetupUrl / *_README_SETUP_URL / readmeSetupUrl → getDocsSetupUrl / *_DOCS_SETUP_URL / docsSetupUrl (only used in this file + its test) so names match reality, and link text project README → documentation.

Type of change

• Bug fix
• New feature
• Refactor / cleanup
• Documentation

Testing

• Tested locally with `bun run dev`
• Linting passes (`bun run lint`)
• Typecheck passes (`bun run typecheck`)

Also ran the modal unit suite: `bun test tests/ui/get-started-modal.test.ts` → 2 pass / 0 fail.

Notes for reviewer

• Both docs URLs resolve to real pages in the docs content tree (`get-started/install`, `guides/self-host`) — no 404s.
• Did not run `bun run dev` (left unchecked above); verified via typecheck, eslint on the changed files, and the modal unit test.
• `IconDoc` already existed in `@/components/shared/icons`; no new icon assets.