Migrate pydocs sphinx docs to project

[taldcroft]

Description

The ancient structure for project documentation was a single RST file in the pydocs repo, so for instance one finds Chandra.Time.rst in that repo.

This PR makes a docs directory and puts the standard sphinx-quickstart structure there with some mods:

• Use the bootstrap-astropy theme. I think this theme is quite clean and modern. It requires that the astropy-sphinx-theme be installed. This is available on conda-forge. We'll need to customize a bit to replace the top-bar astropy logo / link with Ska.
• Use __version__ import from the package as the version and release.
• Adds the GitHub pages extension to enable an automated docs build / publish process. (Note, some repos like eng_archive require Ska data to build the docs.)

Testing

• Passes unit tests on MacOS, linux [not applicable]
• Functional testing: https://cxc.cfa.harvard.edu/mta/ASPECT/tmp/chandra-time/

[javierggt]

I like the idea of moving all to the astropy theme, with Ska mods.