docs: Add London DataFusion meetup to events page

[alamb]

Which issue does this PR close?

• Related to DISCUSSION: Sep 2026 DataFusion Meetup in London #24512

Rationale for this change

There is a DataFusion meetup planned in London on September 30, 2026 — let's advertise it on the community events page.

What changes are included in this PR?

Add an entry for the London meetup to the Community Events section, linking to the discussion and the Luma RSVP page.

Are these changes tested?

Docs-only change; ran ./ci/scripts/doc_prettier_check.sh.

Are there any user-facing changes?

Documentation only.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.30%. Comparing base (dbdc627) to head (cf17a4a).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24521      +/-   ##
==========================================
- Coverage   81.31%   81.30%   -0.01%     
==========================================
  Files        1117     1117              
  Lines      395911   395911              
  Branches   395911   395911              
==========================================
- Hits       321918   321910       -8     
- Misses      55177    55181       +4     
- Partials    18816    18820       +4     

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