CONTRIBUTING: clarify pushing to others' PRs is allowed

While squash-merges have been discouraged from already, they become
impossible to do when the Merge Queue is enabled. In certain scenarios
it was tolerated to use these, however, thus we introduce an acceptable
replacement instead.

By making explicit that pushing to contributors PRs is allowed by
default, it should be easier for committers to do so.

CONTRIBUTING.md

@@ -526,8 +526,15 @@ While this potentially can be understood by reading code, PR discussion or upstr
 Simple package version updates need to include the attribute name, old and new versions, as well as a reference to the release notes or changelog.
 Package upgrades with more extensive changes require more verbose commit messages.
 
-Pull requests should not be squash-merged, as this discards information including detail from commit messages, GPG signatures, and authorship.
-Many pull requests don't make sense as a single commit anyway.
+## Merge conventions
+
+To make changes on commit structure and commit messages or apply simple suggestions, committers are encouraged to [checkout the PR](https://cli.github.com/manual/gh_pr_checkout) and push directly to the contributor's branch before merging.
+Committers will carefully weigh the cost of another review cycle against the feelings of the contributor when pushing to their branch.
+They should also transparently communicate which changes they made.
+If a contributor does not want committers to push to their branch, they must uncheck the "Allow edits and access to secrets by maintainers" box explicitly.
+
+> [!WARNING]
+> Committers: Branches created via `gh pr checkout` can't be pushed with `--force-with-lease`, so do a sanity check before pushing.
 
 ## Code conventions
 [code-conventions]: #code-conventions