Add GitLab CI example with dialyzer cache, refactor sample CI configs into new docs directory

[Nezteb]

The company I currently work for uses GitLab. I spent the last few days setting up dialyxir in our GitLab CI pipeline, so I figured I'd share it with other users. 😄

In order to not overwhelm users with tons of yaml, I also moved the samples into a new docs/ directory.

[jeremyjh]

Thank you!