Refactor monolith .gitlab-ci.yml

I would like to consider refactoring the ~2000-line .gitlab-ci.yml monolith into separate files, to make it easier to manage.

As structure like this might be easier to understand and maintain:

File	Purpose
`.gitlab-ci.yml`	defines stages and includes everything else
`.gitlab/ci/common.yml`	includes common templates (`rules` templates, etc)
`.gitlab/ci/debian.yml`	defines all Debian-related definitions and jobs
`.gitlab/ci/conda.yml`	defines all Conda-related definitions and jobs
`.gitlab/ci/rhel.yml`	defines all RHEL-related definitions and jobs
`.gitlab/ci/compiler.yml`	defines all of the compiler build jobs
`.gitlab/ci/wheel.yml`	defines all of the wheel jobs
`.gitlab/ci/docker.yml`	defines all of the docker jobs

etc etc. I think this can be done in stages, rather than requiring a huge MR that implements everything.