Docs: fix hook name — use wp_abilities_api_categories_init instead of abilities_api_categories_init

[noruzzamans]

Updates docs to use the correct hook name wp_abilities_api_categories_init (was abilities_api_categories_init).
Fixed references and examples in intro.md, php-api.md, and hooks.md.
Docs-only change; no runtime impact. Aligns with WP naming.

[github-actions]

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Co-authored-by: noruzzamans <noruzzaman@git.wordpress.org>
Co-authored-by: gziolo <gziolo@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.39%. Comparing base (d9440e5) to head (a7d35c6).
⚠️ Report is 1 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk     #134   +/-   ##
=========================================
  Coverage     80.39%   80.39%           
  Complexity      178      178           
=========================================
  Files            20       20           
  Lines          1474     1474           
  Branches        120      119    -1     
=========================================
  Hits           1185     1185           
  Misses          289      289           

Flag	Coverage Δ
javascript	94.31% <ø> (ø)
unit	76.31% <ø> (ø)

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