Per #73, reorganized use case documentation and added templates for future use

[briannen]

Documentation only changes, see https://i-wrf--74.org.readthedocs.build/en/74/Users_Guide/usecases.html to review

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[jaredalee]

Only one requested change: Please remove the heading/link for the Generic CONUS "interesting weather". At one point we thought we were going to pursue something like that, but then the Hurricane Matthew case wound up serving as our initial tutorial/demo entry point instead. But once you make those small changes, I approve this PR without needing a second review, and you can go ahead and merge them. Thanks for doing this.

[jaredalee]

Please remove the heading/link for the Generic CONUS "interesting weather". At one point we thought we were going to pursue something like that, but then the Hurricane Matthew case wound up serving as our initial tutorial/demo entry point instead.

[briannen]

@jaredalee I do need another approving review in order to merge.

[jaredalee]

Thanks, @briannen. Good to go now.