Refactor of theme

[nabobalis]

Based on https://github.com/executablebooks/sphinx-book-theme

Fixes https://github.com/sunpy/sunpy-sphinx-theme/issues/136 Fixes https://github.com/sunpy/sunpy-sphinx-theme/issues/36 Fixes https://github.com/sunpy/sunpy-sphinx-theme/issues/54 Fixes https://github.com/sunpy/sunpy-sphinx-theme/issues/95 Fixes https://github.com/sunpy/sunpy-sphinx-theme/issues/93 Fixes https://github.com/sunpy/sunpy-sphinx-theme/issues/52 ToFix https://github.com/sunpy/sunpy-sphinx-theme/issues/21 ToFix https://github.com/sunpy/sunpy-sphinx-theme/issues/29 ToFix https://github.com/sunpy/sunpy-sphinx-theme/issues/94

TODO:

• [x] Working navbar
• [x] Working footer
• [x] Add transparency to the sunpy logo png
• [x] Work out https://sphinx-book-theme.readthedocs.io/en/stable/contributing/subthemes.html maybe
• [X] Work out what issues this PR closes
• [x] Add entry point
• [x] Revert the revert
• [x] Tidy up package again
• [x] Make the colours css variables so other projects can customise them (i.e. ndcube could use it's logo colours not sunpy's)
• [x] Move the footer content out of the layout.html page into it's own template so it is easier to override.
• [X] Check mobile view for header and footer and add HTML/CSS to hide it
• [ ] Fix setuptools' warnings
• [ ] The website, all of the website!
• [ ] Figure out a solution for the toctree issues in the left hand nav bar generated by automodapi (https://github.com/executablebooks/sphinx-remove-toctrees).
• [ ] Do a thorough pass over the colors and CSS after everything is in place
• [ ] Add some sort of CI accessibility test runner: See https://pydata-sphinx-theme.readthedocs.io/en/latest/user_guide/accessibility.html#in-continuous-integration