docs: add Composer-managed installation instructions

[justlevine]

What

This PR adds docs on installing the plugin via Composer and clarifies the language around the intended usage of the plugin.

Why

• Follow up to Change package type from library to wordpress-plugin #66 , per https://make.wordpress.org/ai/2025/09/26/ai-chat-summary-18-september-2025/

  (props: Also reuses and expands on the example given in the PR description by @spenserhale )

[github-actions]

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Co-authored-by: justlevine <justlevine@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.65%. Comparing base (c1b24b8) to head (ccd93fd).
⚠️ Report is 6 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk      #89   +/-   ##
=========================================
  Coverage     86.65%   86.65%           
  Complexity      148      148           
=========================================
  Files            18       18           
  Lines           982      982           
  Branches         92       89    -3     
=========================================
  Hits            851      851           
  Misses          131      131           

Flag	Coverage Δ
javascript	93.04% <ø> (ø)
unit	84.70% <ø> (ø)

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[Copilot]

Pull Request Overview

This PR adds documentation for installing the Abilities API plugin via Composer in WordPress projects and clarifies the usage of plugin dependencies. The changes focus on expanding installation options and improving clarity around intended plugin usage patterns.

• Adds comprehensive Composer installation instructions with repository configuration and installer paths
• Updates language around plugin dependency usage from "best way" to "easiest way"
• Clarifies that plugin dependencies prevent activation but don't auto-install plugins

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