[DOC] Update drop-in hour information on contributing page

[man-shu]

• Closes [DOC] Update drop-in hour information on contributing page #4317

Changes proposed in this pull request:

• replace Nilearn Office Hours are every Friday at 16:00 UTC #2882 with this link.
• add a sentence mentioning the X posts.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.13%. Comparing base (abb80ff) to head (e83be8b).
Report is 27 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4318      +/-   ##
==========================================
+ Coverage   91.85%   92.13%   +0.27%     
==========================================
  Files         144      143       -1     
  Lines       16419    16452      +33     
  Branches     3434     3451      +17     
==========================================
+ Hits        15082    15158      +76     
+ Misses        792      749      -43     
  Partials      545      545              

Flag	Coverage Δ
macos-latest_3.11_test_plotting	91.92% <ø> (+0.07%)	⬆️
ubuntu-latest_3.10_test_plotting	91.92% <ø> (+0.07%)	⬆️
ubuntu-latest_3.11_test_plotting	91.92% <ø> (?)
ubuntu-latest_3.8_test_min	68.81% <ø> (?)
ubuntu-latest_3.8_test_plot_min	91.59% <ø> (?)
ubuntu-latest_3.8_test_plotting	91.88% <ø> (?)

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[bthirion]

LGTM, thx.

[Remi-Gau]

Thanks @man-shu