docs: fix broken links on htmx.org

[SatvikMishra08]

Description

A few links on htmx.org currently go nowhere:

• reference.md: the htmx:sseOpen row links to /events#htmx:sseOpen, but that event isn't on the events page (it's documented by the SSE extension). Pointed it at @/extensions/sse.md#htmx-sseopen, where the heading actually lives.
• essays index: the "Most Recent" heading has a #most-recent anchor link, but the <h3> has no id, so the link doesn't jump anywhere. Added id="most-recent".
• extensions/htmx-1-compat.md: the hx-ws / hx-sse links go to htmx.org/attributes/hx-ws/ and /hx-sse/, which 404 now that those attributes are gone in htmx 2. Pointed them at the v1 docs (v1.htmx.org/attributes/...).
• extensions/idiomorph.md: [swapping](@attributes/hx-swap) is missing the / and .md, so Zola renders it as the relative URL /extensions/idiomorph/@attributes/hx-swap (404). Changed it to @/attributes/hx-swap.md.

Corresponding issue: none (website-only fix)

Testing

Website-only change. I found these with a link check of the live htmx.org and confirmed each new target exists (the htmx-sseopen id on /extensions/sse/, the v1 attribute pages return 200, and content/attributes/hx-swap.md exists).

Checklist

• I have read the contribution guidelines
• I have targeted this PR against the correct branch (master for website changes, dev for
  source changes)
• This is either a bugfix, a documentation update, or a new feature that has been explicitly
  approved via an issue
• I ran the test suite locally (npm run test) and verified that it succeeded (n/a, website only)