Update the suite release process

[izgeri]

What does this PR do?

Our process has evolved slightly from when we first wrote this doc. In this commit, I've updated the process to describe what we currently do.

What ticket does this PR close?

n/a

Checklists

Change log

• [ ] The CHANGELOG has been updated, or
• [x] This PR does not include user-facing changes and doesn't require a CHANGELOG update

Test coverage

• [ ] This PR includes new unit and integration tests to go with the code changes, or
• [x] The changes in this PR do not require tests

Documentation

• [x] Docs (e.g. READMEs) were updated in this PR, and/or there is a follow-on issue to update docs, or
• [ ] This PR does not require updating any documentation

Release PRs

If you are preparing for a release, are the following items complete?

• [ ] The PR title / description clearly state the suite release version.
• [ ] The Jenkins dashboard shows no ongoing build failures for any included components.
• [ ] The PR includes a link to a markdown release notes draft - this can be generated by running:

   summon -p keyring.py   --yaml 'GITHUB_TOKEN: !var github/api_token' \
     ./parse-changelogs \
     -v {suite-version} \
     -t release \
     -o RELEASE_NOTES.md

• [ ] The PR includes PM-approved "What's new" text.