Make preview available through CI

[viroulep]

From @jonatanklosko's comment in #154:

do we also need the PDFs preview? In such case we could look into Netlify as it allows for per-PR preview (i.e. we could generate an index.html listing all the documents, so that one could easily navigate to the PR-specific website build and click around to see the PDFs). Just throwing in the idea, perhaps we're happy as long as the build passes, as Markdown is perfectly readable.

Yes I think it would be awesome to have that!
For instance although the files would build in the linked PR, one of the html tag was weirdly interpreted by pandoc and lead to almost empty pdf!
Having a way to reliably now what files would look like in production would be awesome for both reviewers and contributors!

I actually quickly looked into artifacts but as you seemed to imply it doesn't work per pull request with travis :s
You mentioned Netlify, do you know how hard it would be to setup something like that there?
I guess this mean we would need to migrate the CI for this project there?

[jonatanklosko]

We currently push the build result into the build branch, so maybe we could figure out a way to push to PR-specific branches, although we would still need to figure out cleaning them up somehow. I'm not aware of GitHub solutions dedicated to that (which develop rapidly!), but I just thought of Netlify as I know it has great GitHub integration. In general running Travis build and Netlify build are independent things (i.e. Netlify only does the build part, while Travis may run tests or whatever else), but in our case we would need Travis only for deployment when merging to master. When it comes to configuration, in Netlify you specify the build command and then it automatically starts the process whenever something gets pushed.