Revise squash merge commit title and description guidelines

[jeffpaul]

Updated guidelines for squash merge commit titles and descriptions to improve clarity and efficiency in the merging process.

What?

After discussion starting in #511 (comment), this PR adjusts how squash merge title and description is set.

Copilot summary

This pull request updates the developer documentation to clarify and improve the instructions for writing squash merge commit titles and descriptions. The changes emphasize creating concise, helpful summaries and using changelog entries for commit descriptions.

Documentation improvements:

• Updated the section on squash merge commits in docs/DEVELOPER_GUIDE.md to clarify that the commit title should be a clear summary of the change, and the commit description should be based on the PR's changelog entry or a manually generated summary if none exists.
• Removed outdated instructions about keeping or stripping individual commit messages, and instead focused on using changelog entries for the final commit description.
• Revised the language to stress the importance of this process for easier review and release management.

Why?

How?

Use of AI Tools

None, besides the generation of the commit message/description.

Testing Instructions

Screenshots or screencast

Before	After

Changelog Entry

n/a, tag onto the same changelog entry for #511.

[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>

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.07%. Comparing base (3f8aec5) to head (e0e607c).

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #521   +/-   ##
==========================================
  Coverage      70.07%   70.07%           
  Complexity      1107     1107           
==========================================
  Files             67       67           
  Lines           5367     5367           
==========================================
  Hits            3761     3761           
  Misses          1606     1606           

Flag	Coverage Δ
unit	70.07% <ø> (ø)

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