Deprecate manual instructions

We now have automated tooling which creates a checklist to follow during security releases. The automation should be the single source of truth for our process.

I have opted to move the existing instructions behind an expandable markdown section, as it's possible we may need those instructions if the automation breaks or there are unidentified gaps in the process. At a later point we can remove the instructions altogether.

@ankelly can you review, please? Do we need leadership / release manager input?