[DOCS] Move the release checklist out of the README

[oliverklee]

Part of #1360

[coveralls]

coverage: 97.062%. remained the same
when pulling e4567d2 on task/release-instructions
into bc4e250 on main.

[JakeQZ]

Checks out. Only changes on diff are heading levels and relative paths.

[JakeQZ]

There's supposed to be a 're-run checks' button somewhere, but I'm darned if I can find it. No way the CI should have failed for this, which is only changes to .md files.

[JakeQZ]

The tests failed due to internet connectivity issue.

WHERE HAS THE RE-RUN BUTTON GONE?

[oliverklee]

The tests failed due to internet connectivity issue.

That's what I though as well. But then it turned out the GPG key for the PHPMD PHAR is no longer available:

Warning: Failed with error code 404: Key not found on keyserver

That's why I created #1394.

[oliverklee]

WHERE HAS THE RE-RUN BUTTON GONE?

For me, it's in the actions runner screen, not in the PR:

[oliverklee]

Now that #1394 has been merged and the build (hopefully) is green again, I've rebased this PR. Hopefully the build will be green.

[JakeQZ]

WHERE HAS THE RE-RUN BUTTON GONE?

For me, it's in the actions runner screen, not in the PR:

Finally found it. You have to navigate to the specific workflow run first, by clicking on the title of the associated PR in the list of all workflow runs.