Consider adding CI tests

[DannyBen]

It would probably be beneficial if the repo runs automated build tests on each PR and each push.

Things that can be tested:

• Markdown style - can be done with markdownlint (ruby) or pymarkdownliny (python)
• Broken links - can be done with markdown-helpers (ruby)
• Config folder validation (custom solution)

There are two implementation routes as I see them:

1. Implement all the code needed for testing in this repo directly. This is easier, but probably less desirable, if we want to keep the repo's "documentation only" nature.
2. Implement all the code needed as part of the software-engineering-handbook-tools repo - but then, it must first be converted to an installable pip package, so it is accessible to Travis.