Add machinery and docs for local generation of documentation

[joshuagl]

Some changes to make generating documentation a bit nicer: 1) Add Gemfile to ease generating docs locally 2) Document the process for generating the documentation

[joshuagl]

I will split 2edf93d and c71324c into a separate PR.

I wasn't sure that docs/ was the right place, but we have other maintainer/developer centric docs there so it makes sense. I'll add something and update the PR.

[joshuagl]

I've force-pushed to this PR to keep it focused on locally generating documentation. The other changes have been split into separate PR's #196 and #195.

[MVrachev]

It's a good idea to reference this new doc "Building documentation locally" in the README or docs/index.md file or somewhere else.

Maybe in the manual deployment doc?

[joshuagl]

You're absolutely right, I was trying to avoid doing it until a) the relative links change landed and b) the docs relayout landed - but it would have been useful to mention that here.

I can do that now or (my preference) later, whichever you prefer.

[MVrachev]

You're absolutely right, I was trying to avoid doing it until a) the relative links change landed and b) the docs relayout landed - but it would have been useful to mention that here.

I can do that now or (my preference) later, whichever you prefer.

Okay, let's do it later then. :))

[joshuagl]

Travis failed because Yarn timed out fetching a package, I've restarted the build.