Clarify release-process.md regarding distro package builders

Summary:
Make distro package building a separate step in the process. It is
logically distinct from Gitian building, and in the past notifying
the PPA builder has often been missed.

Move to after Github release step. Feedback from package builders
indicates they prefer to begin building their packages after the rest
of the release is complete.

Add AUR and Docker image building.

Test Plan:
Follow the steps in the checklist with every release, fix
problems as they arise, repeat.

Reviewers: deadalnix, Fabien, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, jasonbcox, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2294