[LOW-280] Manual changes during release process

The release process steps as currently documented at https://developer.skao.int/en/latest/tools/software-package-release-procedure.html#release-steps do not allow for manual changes that must be made in order to effect a version update. The process tooling automatically handles

• Updating the .release file
• Updating the package version in pyproject.toml

but it does not automatically handle (for example)

• Updating the release variable in docs/conf.py;
• Updating the package's __version__ attribute;
• Updating a human-readable CHANGELOG file

These tasks need to be done manually, and can actually be safely performed between the current steps 4 and 5.

This MR updates the "Release steps" section to provide this guidance.