Update release process to guide team practice

[EoinShaughnessy]

Addresses #2175 (Map release process and identify pain points).

We recently had a retro to identify pain points in our release process.

This pull request contains revised steps, based on team feedback, to the doc for our release process. For example, that no major changes should go into a release after an agreed cutoff-point.

Hopefully these revisions will make our future releases run more smoothly.

[EoinShaughnessy]

Do we need to change master to main in this content? Or can that wait till we've completed the branch-renaming work?

[vanitabarrett]

@EoinShaughnessy I think we can do that separately - it might take us a while to do the rename and we probably don't want to update the docs until it's done, so I wouldn't want us to block this PR 🙂

[hannalaakso]

Thanks @EoinShaughnessy 🎉 I think these changes will really help us to streamline the release process. Noticed a couple of small things that just need tidying up.

[EoinShaughnessy]

@hannalaakso Thanks for the excellent review! I think I've actioned your feedback now. If things look ok, I'll add in the final commits and then get to squashing.

[hannalaakso]

Thanks for addressing all my comments @EoinShaughnessy, this looks good to me 🙏 I've given it an approval but squashing the commits before it's merged sounds great.