Add TravisCI build script.

Created by: jm4rtin

This will try to build the HTML and PDF documentation. It runs the build in "nit-picky" mode and converts warnings into errors. It actually fails the PDF generation due to the SVG used in the section about OBI. Not sure what you would want to do about that.

It would be good to add this as a check for PRs.

Example build: https://travis-ci.com/github/emmicro-us/core-v-docs/builds/172578562