Document the internal release process in the handbook

Context

&1201 (closed) adds an internal release process to the GitLab release strategy to remediate single tenant SaaS instances within the SLAs

As with any other GitLab release process, it should be documented in the handbook.

Links to reference in the handbook

• Internal procedures for engineers and release managers created in #20730 (closed)
• Related handbook pages like https://handbook.gitlab.com/handbook/engineering/architecture/design-documents/internal_releases/

Exit Criteria

• Expand the GitLab release page with the internal release process https://handbook.gitlab.com/handbook/engineering/releases/
• Create a separate in-depth page similar to https://handbook.gitlab.com/handbook/engineering/releases/patch-releases/ for internal releases