Redesign online documentation using sphinx-material

This PR introduces a revamp of the online documentation using sphinx-material in place of the current sphinx_bootstrap_theme.

An example can be viewed by LIGO.ORG account holders here:

https://ldas-jobs.ligo.caltech.edu/~duncan.macleod/gwpy/sphinx-material/

I would appreciate feedback on the new design from anyone, especially @gwpy/maintainers.

EDIT: fixes #1327 (closed)