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 85.69%. Comparing base (8e0b08a) to head (d4ac11b).

Additional details and impacted files

@@            Coverage Diff            @@
##              trunk      #89   +/-   ##
=========================================
  Coverage     85.69%   85.69%           
  Complexity      103      103           
=========================================
  Files            16       16           
  Lines           776      776           
  Branches         86       85    -1     
=========================================
  Hits            665      665           
  Misses          111      111           

Flag	Coverage Δ
javascript	92.66% <ø> (ø)
unit	82.97% <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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