Link to FAQ from settings section

[Gilmoursa]

Summary

Fixes #2442.

Users frequently open support topics confused about why their uploads aren't generating AVIF/WebP files. The FAQ in readme.txt already explains the primary reason (the plugin discards converted images that would be larger than the original), but there was nothing in the admin UI pointing users there.

This adds a brief note with a link to the plugin FAQ on WordPress.org in the Modern Image Formats settings section on Settings > Media, so users can find the explanation without having to search the support forums.

The section previously used __return_empty_string as its callback (no description rendered). This replaces it with a dedicated callback that outputs a single sentence linking to the FAQ.

Changes

• plugins/webp-uploads/settings.php: replace __return_empty_string section callback with webp_uploads_modern_image_format_settings_section_callback() which renders a FAQ link
• plugins/webp-uploads/tests/test-settings.php: add test asserting the callback outputs the FAQ URL and an anchor tag

Test plan

• Navigate to Settings > Media with the Modern Image Formats plugin active — confirm a note with a link to the FAQ appears under the "Modern Image Formats" section heading
• Confirm the link points to https://wordpress.org/plugins/webp-uploads/#faq
• Run the unit tests: composer test:integration -- --filter test_webp_uploads_modern_image_format_settings_section_callback

[github-actions]

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Co-authored-by: Gilmoursa <gilmoursa@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: b1ink0 <b1ink0@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.64%. Comparing base (879e67b) to head (c7e6f1d).

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #2480      +/-   ##
==========================================
+ Coverage   69.11%   69.64%   +0.53%     
==========================================
  Files          90       90              
  Lines        7726     7736      +10     
==========================================
+ Hits         5340     5388      +48     
+ Misses       2386     2348      -38     

Flag	Coverage Δ
multisite	69.64% <100.00%> (+0.53%)	⬆️
single	35.44% <0.00%> (-0.05%)	⬇️

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[westonruter]

This adds the paragraph after the Modern Image Formats heading: