Update documentation step for the release process

[ADubhlaoich]

Proposed changes

User-facing documentation was migrated to the main NGINX documentation repository. This commit changes the instructions and tooling configuration to reflect what has changed.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

[ADubhlaoich]

Beyond the scope of updating the release process documentation, I'm gonna double-check if the guidance around contributing mentions the documentation repository and how branch management works. I

t's relatively straightforward, but still crucially important - especially since only the team will be involved with releases, whereas anyone could be contributing to documentation at any time.

[ADubhlaoich]

The documentation repository will be protecting release branches with the pattern name <product>-release-<version>: noting it here is not necessary within the scope of the release process, but means we can assert exactly how the new release branch should be named.