Publish SASS API docs

[james-nash]

Is your feature request related to a problem? Please describe.
We added SASS docs to gravity-ui-web recently. However, the only way to see them is to check out the code, build the docs locally and open them in your browser. We don't currently publish them on the web, but we should. :-)

Describe the solution you'd like

1. Decide what the URL(s) should be.
   • Perhaps the easiest thing would be to piggy-back on the existing http://style[-*].buildit.digital/ domain where the pattern library lives and just add a directory. E.g. http://style-next.buildit.digital/sassdoc/
   • A variation on the above would be to also move the pattern library into a sub-folder and add a simple landing page that links to both:
     • http://style-next.buildit.digital/: Simple landing page with links to pattern library & SASS docs
     • http://style-next.buildit.digital/patterns/: Pattern library
     • http://style-next.buildit.digital/sassdoc/: SASS docs
   • Alternatively, setup a dedicated subdomain (and associated S3 bucket). E.g. gravity-sassdocs.buildit.digital/?
2. Decide whether we should also publish a dev build of the SASS docs (which includes internal APIs). So, we might have both .../sassdoc/ and .../sassdoc-dev/ or similar.
3. Update .travis.yml to:
   • Run npm run docs (and possibly also npm run docs:dev)
   • Upload the output from dist/docs/sass/ (and possibly also dist/docs/sass-dev/) to the appropriate location on Amazon S3