Documentation Site Overhaul

[RHammond2]

This a draft for an ongoing PR to overhaul the documentation site build for Read the Docs. The main changes are as follows:

• Using the Pydata Sphinx Theme and updating sphinx/conf.py to format the site as expected
• Removing sphinx/requirements.txt in favor of using setup.py with pip install -e "openoa[docs]" workflow to setup the build dependencies
• Fixing the formatting and layout contained in the main pages so that heading syntax is used properly and consistently
• Rewriting the PlantData page to be better show what information is expected from the user
• Incorporation of the rewritten examples notebooks

This PR should be the last to be merged in the current list so that the current batch of updates can be incorporated into the code base, and the documentation updated accordingly.

Note: to build these locally, just run from within the top-level of openoa/ (essentially the same instructions as before):

• pip install ".[docs'}
• cd sphinx
• make html
• open _docs/html/index.html in the browser of your choice

[jordanperr]

• [ ] Inline comments
• [ ] I recommend reverting to sphinx/requirements.txt, or at least retaining a copy, since this is what readthedocs.org is configured to read.
• [x] It might be clearer to combine the PlantData, Utils, and Analysis tabs into a single "API" tab.
• [ ] The final cell in the example notebook appears to have an error.

[CLAassistant]

All committers have signed the CLA.

[RHammond2]

Change the curtailment and availability units to kWh from GWh