update release process documentation

[stephprince]

Motivation

Added instructions for making the pre-release PR and going through the template checklist. I also moved some of the prerequisites to this new section.

Checklist

• Did you update CHANGELOG.md with your changes?
• Have you checked our Contributing document?
• Have you ensured the PR clearly describes the problem and the solution?
• Is your contribution compliant with our coding style? This can be checked running flake8 from the source directory.
• Have you checked to ensure that there aren't other open Pull Requests for the same change?
• Have you included the relevant issue number using "Fix #XXX" notation where XXX is the issue number? By including "Fix #XXX" you allow GitHub to close issue #XXX when the PR is merged.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.91%. Comparing base (d3ac4b9) to head (db8fb40).
Report is 17 commits behind head on dev.

Additional details and impacted files

@@           Coverage Diff           @@
##              dev    #1928   +/-   ##
=======================================
  Coverage   91.91%   91.91%           
=======================================
  Files          27       27           
  Lines        2648     2648           
  Branches      691      691           
=======================================
  Hits         2434     2434           
  Misses        141      141           
  Partials       73       73           

Flag	Coverage Δ
integration	72.80% <ø> (ø)
unit	83.98% <ø> (ø)

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