docs: repoint the two links that 404

[lesnik512]

Summary

Two links in this repo 404. Both are visible to readers: they are in the README's opening line and
its "Already using Postgres?" callout, so they are also on the PyPI page as of 0.4.2.

• https://faststream.airt.ai now returns GitHub Pages' "Site not found". FastStream's docs live at
  https://faststream.ag2.ai/ (verified: loads, FastStream 0.7.5).
• https://faststream-outbox.readthedocs.io/en/latest/usage/timers/ 404s, and faststream-outbox does
  not publish to Read the Docs at all — its mkdocs.yml sets
  site_url: https://faststream-outbox.modern-python.org/, and the page exists at
  https://faststream-outbox.modern-python.org/usage/timers/ (verified: loads, titled "Timers").

Found while verifying the rendered PyPI pages after the 0.4.2 release. The docs site picks this up on
merge; the PyPI page picks it up on the next release.

Changes

• README.md: both links repointed.
• docs/index.md: the same two links in the mirrored intro and tip block.
• CONTEXT.md: the FastStream link in the opening description.

Checklist

• Lint and format pass (ruff) — no Python touched
• Type check passes (ty) — no Python touched
• Tests pass and new behavior is covered — docs-only, no behavior
• Build succeeds (uv build) if packaging or build config changed — not touched; just docs-build (mkdocs --strict) passes locally
• Repo metadata stays consistent across the three surfaces — not touched