update docs for release

[jmccreight]

• User visible changes (including notable bug fixes) are documented in whats-new.rst
• Refresh/polish notebooks.
• General improvement of the RDT pages.

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (be79e76) 79.66% compared to head (a7c9f85) 79.66%.
Report is 21 commits behind head on develop.

❗ Current head a7c9f85 differs from pull request most recent head d736df5. Consider uploading reports for the commit d736df5 to get more accurate results

Files	Patch %	Lines
pywatershed/base/process.py	88.88%	5 Missing ⚠️
pywatershed/atmosphere/prms_atmosphere.py	80.00%	3 Missing ⚠️
pywatershed/base/control.py	96.55%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #257   +/-   ##
========================================
  Coverage    79.66%   79.66%           
========================================
  Files           50       50           
  Lines         6614     6624   +10     
========================================
+ Hits          5269     5277    +8     
- Misses        1345     1347    +2     

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