Some small tutorial improvements

[bhazelton]

Description

Some tutorial tweaks to separate out examples for the various functionality (before some examples showed a bunch of unrelated functionality in the same example). Also added some more text to explain what was going on and made a few figures prettier.

Also ensure that plotting commands are run in the doctests to make sure they work (there have been bugs in the pyuvdata tutorial that we didn't catch because they were in plotting commands that weren't run in the doctests). This required some updates to the docs requirements.

Note: This is best reviewed by looking at the RTD build.

Motivation and Context

Better docs always help :).

Checklists:

Documentation Change Checklist:

• I have read the contribution guide.
• My code follows the code style of this project.
  Documentation change checklist:
• Any updated docstrings use the numpy docstring format.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (1cae5da) 99.95% compared to head (fc40fc3) 100.00%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #226      +/-   ##
===========================================
+ Coverage   99.95%   100.00%   +0.04%     
===========================================
  Files           5         5              
  Lines        2134      2134              
  Branches      608       608              
===========================================
+ Hits         2133      2134       +1     
+ Misses          1         0       -1     

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