Migrate docs.stackstorm.com to GH static pages

[arm4b]

Currently, https://docs.stackstorm.com/ are deployed to AWS S3 on every build.
Migrating to native GH static pages unties docs from AWS and would improve the transparency via git, security, and maintenance.
We had a good experience with Github Static pages as a great alternative so far where additional infra would be redundant.

Depends on #1095
Relevant story for apidocs: StackStorm/st2apidocs#16

[cognifloyd]

I think rtfd.io (readthedocs.org) would be ideal for docs.stackstorm.com as they already have the infra to deal with sphinx. Using a CNAME record we can continue using docs.stackstorm.com as well.

[arm4b]

readthedocs.org is really nice, but it's still 3rd party service that will require another new set of credentials and deployment.

With security in mind, what git + GH static pages provide us is native integration and transparency. The entire pipeline doesn't escape Github and everyone can track if the docs.stackstorm.com state is indeed in sync and not modified in the middle or something.