Update release process for missing parts

The release process documentation should:

• Mention that a release plan is needed.
• Explicitly mention that release notes documentation should be checked to ensure it is up-to-date.
• Mention the mandatory release review, steps involved, etc. See also #9 (closed).
• For GitLab milestone, mention that it should refer to the official release record, and vice versa. See e.g. v0.1 and v0.2 GitLab milestones.