Generate rust documentation on CI and publish to gh-pages automatically

[victorporof]

Signed-off-by: Victor Porof victor.porof@gmail.com

A very simple start for generating documentation automatically.

Here's how it looks like: https://victorporof.github.io/mentat When this PR is merged, this link will also point to the rust API documentation: https://mozilla.github.io/mentat

The gh-pages branch would look like this: https://github.com/victorporof/mentat/tree/gh-pages

This PR implements the following:

1. Upon successful test runs on the master branch (and not for pull requests), run cargo doc to generate documentation.
2. If that succeeds, create an index.html entry-point for the projects's gh-pages website that redirects to the generated documentation for the top-level mentat crate.
3. Copy the CI-generated documentation and the index into the gh-pages branch, overwriting everything that was previously there.

A link to the documentation is also added to the README.

The importing of the generated documentation into the gh-pages branch is done with a very simple ghp-import python script that I came across. In the long term, it's likely that we'll want to roll out our own, especially since we want to provide more than just Rust API documentation. However I think this is fine for now.

I've added a GitHub token for allowing pushing to the repo from the CI machine: https://travis-ci.org/mozilla/mentat/settings It follows the recommendations from https://docs.travis-ci.com/user/deployment/pages/#Setting-the-GitHub-token

[victorporof]

Thanks @fluffyemily! Yeah, the plan is to generate other docs on CI in this manner as well, and push from CI back into the gh-pages like in this PR.

Will be looking into https://github.com/mozilla/mentat/pull/765 next.

We’ll likely want a different index however, likely what’s in the docs directory right now, linking to the appropriate generated documentation, but also including our tutorials, getting started guides etc.

[victorporof]

Thanks for the quick review. Landing.