Consider hosting RTD docs locally

[MarkDavidson]

Currently, YETI has documentation hosted on readthedocs.org. This issue is to consider whether having them also hosted locally by YETI is a good idea or not.

Pros:

• The YETI documentation would be available "offline" - e.g., if you deploy YETI in a place without internet connectivity you can still get at the documentation. This pro can also be achieved by mentioning the offline documentation that readthedocs makes available.

Cons:

• Devs will forget to re-generate the documentation at least some of the time, resulting in out-of-date documentation in YETI's local copy

Open Questions:

• Would django-taxii-services docs make sense to include, since django-taxii-services has all of the TAXII functionality