Revise readme.txt for new version

[felixarntz]

This PR revises the contents of readme.txt to better reflect the purpose and behavior of the new plugin version in trunk. It takes inspiration from the existing readme.txt from the legacy-plugin, as well as the existing README.md file from trunk.

Other than the readme.txt, a few related miscellaneous changes are included:

• The README.md introduction is updated to use the same content as the new readme.txt.
• The plugin short description is updated throughout to be consistent with the short description in readme.txt.
• All tests plugins used for unit tests, which were previously using the old plugin description, have been updated to use a dummy description instead.

Fixes #300.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (db28ffe) 39.49% compared to head (466e630) 39.49%.

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk     #323   +/-   ##
=======================================
  Coverage   39.49%   39.49%           
=======================================
  Files          43       43           
  Lines        2028     2028           
=======================================
  Hits          801      801           
  Misses       1227     1227           

Flag	Coverage Δ
feature	39.49% <ø> (ø)

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Files	Coverage Δ
plugin-check.php	39.28% <ø> (ø)

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[joemcgill]

@felixarntz I've left some editing feedback. Nothing that I think needs to be blocking, but could help with readability and consistency while we're thinking about it.

[felixarntz]

Thanks @joemcgill, I've addressed your feedback. Could you please take another look?

[joemcgill]

Thanks, @felixarntz!