Improve docs of the GitLab docs site release process

What does this MR do and why?

Move some docs release instructions from the gitlab-docs project's own documentation to the issue template for this task. This MR is part of an ongoing effort to move all, or almost all, of the instructions for creating a GitLab docs release.

In this MR, the improvements/changes:

• Reference the "Create a stable branch..." instructions in the issue's description instead of the releases.md file.
• Ensure the "Create a stable branch..." instructions in the issue's description match the previous content of the releases.md file.
• Remove the "Create a stable branch..." section from the releases.md file.
• Update the docs release readiness step to check all 4 projects instead of checking for the candidate commit announcement.

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