Clean up and reorganize

[dopplershift]

This is an attempt to reorganize the existing content into some broad categories so we can remove the "under construction" moniker and have something we can point people to (since the previous home for this content has horribly bit-rotted).

I've currently chosen:

• Skew-T
• Convective Calculations
• Synoptic Analysis
• Specialty Plotting
• Simplified Plotting (Declarative)

I'm open to suggestions, but not a drawn out discussion. It's way past time we get this up and back out to the world.

[github-actions]

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[dopplershift]

Ok, this is about ready. The only warning is:

• /home/runner/work/metpy-cookbook/metpy-cookbook/README.md:: WARNING: toctree contains reference to nonexisting document 'notebooks/how-to-cite' [etoc.ref]
• We also get a HTTPError: HTTP Error 308: Permanent Redirect for some reason loading station information in the DIFAX example