Split out Inbox Release Management and update

[legoktm]

Move to its own file now that the process is (temporarily) diverging from the Workstation.

Incorporate the new release schedule, the freeze on main, and lack of release branches. Add notes that apt-test and apt-prod PRs will soon be automated, and flag that the preflight instructions are already out of date.

Checklist

This change accounts for:

• local preview of changes beyond typo-level edits

[nathandyer]

This is mostly looking good.

There's an issue with the indentation of all the code blocks that will need to be corrected, but that's the only substantive issue I'm seeing.

Also just a note: in the main securedrop docs repo we've moved away from hard line breaks, but I'm not applying that requirement here because we haven't actually established that as a requirement in this repo, and also because we'll need to eliminate them in all the existing docs before we can apply this uniformly to new PRs.

[nathandyer]

There's an indent-level issue with all the code blocks here.

As it stands, it's currently putting all the code blocks inside a quote block. It's also considering them nested under the parent rather than the individual bullet items.

See the before:

Vs after:

Suggested change

	.. code-block:: sh
	git clone git@github.com:freedomofpress/securedrop-client.git
	git clone git@github.com:freedomofpress/securedrop-builder.git

I defer to @legoktm for where it makes sense to nest it inline with the step vs with the parent, the above suggestion is just one example of how this could be fixed.