Convert build to a Sphinx site and minor updates to docs

[choldgraf]

This PR does a few main things:

• Updates the repository to be a Sphinx site instead of a Jupyter Book. This gives us a bit more control over things and lets us preview the documentation on ReadTheDocs. It retains the same structure and look as if we used Jupyter Book though.
• Adds links to a few JEPs that were not added to the table of contents when they were accepted.
• Adds numbering to these JEPs to make them more in-line with our other JEPs
• Adds the current active JEP in Jupyter Notebook version 7 #79 to our README
• Adds instructions to our README for how to build the docs locally
• Adds a noxfile.py so that we can automate building docs via nox
• Removes the CircleCI build, since we are now building the docs with ReadTheDocs previews

We can now preview the built docs in PRs, like so: https://jupyter-enhancement-proposals--80.org.readthedocs.build/en/80/README.html

[blink1073]

Thanks!

[blink1073]

Merging since this is an admin-only change.

[choldgraf]

(we can disable docs builds on CircleCI if this gets merged, though I do not have permissions to do so)