Add custom experiment reference docs

[yusufhay]

What?

Closes #184

Adds a documented custom experiment reference for developers extending the AI plugin.

Why?

The repository already includes example experiment code, but it is not surfaced as an easy-to-follow reference for plugin developers. This makes the extension path harder to discover than it needs to be.

How?

• add a new docs/experiments/custom-experiment-reference.md page
• link that reference from the developer guide table of contents and resources section
• add a short pointer from the main README developer experience section
• clearly mark the example as reference/demo code and point readers to the in-repo Example_Experiment implementation

Use of AI Tools

AI assistance: Yes
Tool(s): Codex
Model(s): GPT-5
Used for: drafting the initial documentation update and PR text; final file changes were reviewed and committed by me.

Testing Instructions

1. Open docs/experiments/custom-experiment-reference.md and confirm the example explains how to register a custom experiment.
2. Open docs/DEVELOPER_GUIDE.md and confirm the new reference is linked from the table of contents, experiment creation section, and resources section.
3. Open README.md and confirm the developer experience section links to the new reference page.

Screenshots or screencast

Not applicable.

Changelog Entry

Development Update - Add custom experiment reference documentation for developers extending the plugin.

[github-actions]

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Co-authored-by: yusufhay <yusufmudagal@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.16%. Comparing base (afc7cb4) to head (32ca142).
⚠️ Report is 437 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #499      +/-   ##
=============================================
+ Coverage      62.64%   69.16%   +6.52%     
- Complexity       681      981     +300     
=============================================
  Files             49       63      +14     
  Lines           3515     4648    +1133     
=============================================
+ Hits            2202     3215    +1013     
- Misses          1313     1433     +120     

Flag	Coverage Δ
unit	69.16% <ø> (+6.52%)	⬆️

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