docs: Add narwhals-datafusion to integrations list

[s5dsn-eqee]

Which issue does this PR close?

N/A

Rationale for this change

The introduction page invites projects using DataFusion to add a link.

What changes are included in this PR?

Adds narwhals-datafusion, a plugin that runs the Narwhals dataframe API on DataFusion, to the Integrations list.

Are these changes tested?

Docs only.

Are there any user-facing changes?

No.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.93%. Comparing base (140c7c5) to head (eddaaaf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25387      +/-   ##
==========================================
- Coverage   81.93%   81.93%   -0.01%     
==========================================
  Files        1136     1136              
  Lines      429152   429152              
  Branches   429152   429152              
==========================================
- Hits       351633   351623      -10     
- Misses      56475    56483       +8     
- Partials    21044    21046       +2     

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