Improvements for release_process.md

[MeGaGiGaGon]

While cutting 25.9.0, I ran into a few snags, so this PR smooths out the issues I ran into in the docs:

• Minor typo fix
• Since the stable branch is updated by the release workflow, git diff origin/stable CHANGES.md should always work, which is much easier than having to find a commit hash
• The example PR given had the new template already applied, which confused me. I eventually used the 25.1.0 PR as a reference instead
• GitHub appears to now use the last commit message as the release message instead of the tag name, so it will need to be entered manually going forwards
• There looks to be a check to prevent all pushes directly to main, so a PR has to be made

Description

Checklist - did you ...

• [N/A] Implement any code style changes under the --preview style, following the
  stability policy?
• [N/A] Add an entry in CHANGES.md if necessary?
• [N/A] Add / update tests if necessary?
• Add new / update outdated documentation?