Improve documentation of the release process.

[n2ygk]

Fixes #

Description of the Change

Update contributing docs and tox.ini to match how releases are now done.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #1063 (c72a7e0) into master (c9ce19c) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head c72a7e0 differs from pull request most recent head c920b2c. Consider uploading reports for the commit c920b2c to get more accurate results

@@            Coverage Diff             @@
##           master    #1063      +/-   ##
==========================================
+ Coverage   96.60%   96.64%   +0.03%     
==========================================
  Files          31       31              
  Lines        1739     1756      +17     
==========================================
+ Hits         1680     1697      +17     
  Misses         59       59              

Impacted Files	Coverage Δ
oauth2_provider/settings.py	100.00% <0.00%> (ø)
oauth2_provider/views/base.py	97.79% <0.00%> (+0.01%)	⬆️
oauth2_provider/models.py	98.69% <0.00%> (+0.01%)	⬆️
oauth2_provider/views/mixins.py	99.24% <0.00%> (+0.02%)	⬆️
oauth2_provider/oauth2_validators.py	93.56% <0.00%> (+0.02%)	⬆️
oauth2_provider/decorators.py	97.67% <0.00%> (+0.11%)	⬆️
oauth2_provider/http.py	85.00% <0.00%> (+0.78%)	⬆️

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[auvipy]

the explicitness is much better with this.

[auvipy]

should all the test matrix triggered with doc changes? can we improve the CI and avoid running all tests for docs changes only?