docs(guide): remove the Sequin section from the syncing guide

[alangmartini]

What

Removes the ### Using Sequin section (46 lines) from
docs-site/content/guide/syncing-data-into-typesense.md.

Why

Sequin moved into maintenance mode after the team was acquired by Notion, so we should
not keep recommending it as the real-time Postgres → Typesense path:

• Last commit on sequinstream/sequin: 2026-02-23, zero commits in the ~5.5 months since.
• Last release: v0.14.6, published 2026-02-18.
• sequinstream/sequin#2111, "Update documentation to reflect maintenance mode", merged
  2026-02-09, body: "Removing some outdated documentation from the transition to
  maintenance mode post-acquisition." The maintainers said it themselves; this is not
  inferred from silence.
• Two open bugs in the Typesense sink specifically, both with zero comments:
  sequinstream/sequin#2153 (sink hardcodes action=emplace, so omitted fields persist in
  the index) and sequinstream/sequin#2146 (sink fails on IPv6-only Fly.io .internal hosts).

The repo is not archived and the README still reads as a normal active project, which is why
this keeps catching users out.

What readers get instead

The guide keeps every other sync strategy it already documented, polling on updated_at,
change listeners / CDC, ORM hooks, and Airbyte (which supports Postgres as a source and
Typesense as a destination). Airbyte is now the only third-party sync tool the guide points to.

Notes for review

• Pure deletion; no rewording of surrounding sections, no new content.
• The section sat under ## Sync real-time changes, which still has Using the API and
  High-volume writes.
• [[toc]] is generated, so no manual table-of-contents edit is needed.
• External links to ...syncing-data-into-typesense.html#using-sequin will land at the top of
  the page after this ships. If we'd rather not break them, the alternative is a short
  deprecation note in place of the section instead of a deletion, say the word and I'll swap it.