Motivation:

Up until now, Jazzy docs were generated on a private CI runner hosting macOS - which doesn't make CI portable as it requires that runner to always be available.

Modifications:

- Change: CI config to use a Linux Jazzy Docker image for portability
- Change: Doc theme templates to not reference GitHub

Result:

Jazzy docs should be able to be generated at any time, as the CI infrastructure is using GitLab's cloud solution with a Docker image