Use this repo as the basis for a docs template repo once i18n workflow is in place

[robredpath]

In the ODS GitHub org we have a sphinx-base repo that builds with our organisation-standard set of Sphinx stuff.

We should adopt that pattern in IATI. We're using the validator docs as a bit of a testbed for development, but once that's calmed down a bit we should make a base repo that we can use for all future docs sites that follow this pattern.