Add missing experiment documentation pages and cross-links

[dilipom13]

What?

Closes #221

Add missing experiment documentation pages for:

• Title Generation
• Experiment Framework
• Multi-Provider Support

Also, cross-link these new docs from relevant documentation entry points.

Why?

Several experiment-related capabilities existed in code and user-facing docs lists, but dedicated pages under docs/experiments/ were missing.
This made documentation inconsistent and reduced discoverability for contributors and users trying to understand these features and extension points.

How?

• Added new documentation files:
  • docs/experiments/title-generation.md
  • docs/experiments/experiment-framework.md
  • docs/experiments/multi-provider-support.md
• Linked the new docs from:
  • README.md (Current Features section)
  • readme.txt (Current Features section)
  • docs/DEVELOPER_GUIDE.md (Additional Resources)

Use of AI Tools

AI assistance: No

Testing Instructions

1. Verify the following files exist and render correctly:
   • docs/experiments/title-generation.md
   • docs/experiments/experiment-framework.md
   • docs/experiments/multi-provider-support.md
2. Open README.md and confirm links to the 3 new docs are present in Current Features.
3. Open readme.txt and confirm links to the 3 new docs are present in Current Features.
4. Open docs/DEVELOPER_GUIDE.md and confirm the 3 new docs are listed in Additional Resources.
5. Click each added link to confirm it resolves correctly.

Screenshots or screencast

Not applicable (documentation-only changes).

Changelog Entry

Development Update - Add missing experiment documentation pages and cross-link them from project docs/readmes.

[github-actions]

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Co-authored-by: dilipom13 <dilip2615@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: dhrumilkumbhani <dhrumilk@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.21%. Comparing base (b9f820d) to head (df79750).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #475   +/-   ##
==========================================
  Coverage      71.21%   71.21%           
  Complexity      1151     1151           
==========================================
  Files             67       67           
  Lines           5565     5565           
==========================================
  Hits            3963     3963           
  Misses          1602     1602           

Flag	Coverage Δ
unit	71.21% <ø> (ø)

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[dhrumilkumbhani]

1. The experiment enqueues src/experiments/title-generation/index.tsx on post editing screens.

This is my suggestion—if it looks good, we can update it like this.

[dhrumilkumbhani]

When higher-priority preferences are unavailable.

This is my suggestion—if it looks good, we can update it like this.

[dilipom13]

@dkotter, conflict issue resolved, and updated the branch.

[jeffpaul]

Removing title gen updates and accepting what's in develop from #501.