Reuse pygplates' doc design for gplately doc

[michaelchin]

Hi @GPlates/gplately-dev

I would like to propose using pygplates' doc design for gplately doc.

1. the pygplates doc looks very nice.
2. John has experience in how to do it
3. the doc from the same software family should have consistent style

[jcannon-gplates]

This sounds good 👍

I mention a little about the potential chapter structure here.

And we can start on that structure even before moving to Sphinx (ie, before making the GPlately docs have a similar style to the pyGPlates docs).