Add/contribution criteria

[jeffpaul]

What?

This PR adds some docs (and tidies up existing docs in a couple places) to generally cover questions that @justlevine asked in #75. While I don't expect we'll need to consider how an Experiment graduates OUT of the AI Experiments plugin towards core in the near-to-medium future, I still went ahead and tried to describe how that likely would work. Documenting how a new Experiment lands in the AI Experiments plugin definitely felt worthwhile, so that's here too. Finally I tried to make sure the repo properly covered items as outlined in @desrosj's Criteria for Creating or Migrating Repositories under the WordPress GitHub Organization post (albeit delayed in updating for that but its done now in any case).

Closes #75.

Copilot Summary

This pull request improves documentation for the AI Experiments plugin by adding a new guide describing the experiment lifecycle, updating references to this new documentation throughout the project, and enhancing the README and release instructions for clarity and accuracy.

Documentation improvements:

• Added a new EXPERIMENT_LIFECYCLE.md document detailing how experiments are proposed, reviewed, included in the plugin, and potentially graduate to WordPress core.
• Updated CONTRIBUTING.md and docs/DEVELOPER_GUIDE.md to reference the new experiment lifecycle documentation, helping contributors understand the process for proposing and evolving experiments. [1] [2]

README and feature descriptions:

• Expanded the README.md with a "Current Features" section listing and describing the main plugin features, and added a link to the Credits file for contributor recognition. [1] [2]
• Clarified the description of the image generation feature in both README.md and readme.txt to specify that it creates a feature image from post content in the block editor. [1] [2]

Release instructions and minor fixes:

• Updated links in docs/RELEASE_INSTRUCTIONS.md to use the correct GitHub organization casing and URLs, ensuring consistency and accuracy in release documentation. [1] [2]
• Made a minor formatting cleanup in docs/TESTING_REST_API.md.

Why?

How?

Testing Instructions

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

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

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[jeffpaul]

@justlevine anything else you think should be called out in the changes here or otherwise good to go?

[justlevine]

Content-wise I'm feeling good, flagged a numbering nit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 56.76%. Comparing base (04c2df4) to head (e614d11).
⚠️ Report is 41 commits behind head on develop.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #219   +/-   ##
==========================================
  Coverage      56.76%   56.76%           
  Complexity       505      505           
==========================================
  Files             32       32           
  Lines           2565     2565           
==========================================
  Hits            1456     1456           
  Misses          1109     1109           

Flag	Coverage Δ
unit	56.76% <ø> (ø)

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