Update docs: Fix links, Add aircraft plots, Update yaml descriptions, Update observation description

[rschwant]

This updates the docs and will close the following issues:

• Update old project board links on ReadTheDocs
  #224
• Update documentation for all observational and model datasets
  #205
• Update YAML options in Scatter Density plot
  #293
• Need to update aircraft plot descriptions on ReadTheDocs
  #294
• Need to add aircraft plot yaml descriptions to the yaml appendix
  #295
• I also updated the aircraft examples, to make the plots look nicer, so only 2 day pairing and bigger font sizes.

During this process, I realized that some of the YAML options in the aircraft plots are not fully connected but still listed in the example YAML files and or need some cleaning up. I created a new issue for this that we will address in Version 2. All YAML options described on the ReadTheDocs YAML appendix description seem to be working as specified and represent the base options available so far for these plots. Clean up aircraft plotting example YAML files #341.

[rschwant]

I just added a note for the CONUS extent to this PR too. This will close this PR too: domain_type == 'all' behavior
#140

I also created a new issue and put it in version 2 to generalize this extent feature: #343. Generalizing this now since it is used in several examples seems to rushed. Better to document current capabilities and we can advance this further in version 2.

[zmoon]

A few initial comments

[rschwant]

This is ready for review when you all have time.