[Docs] Document CI System (#14656)

Fixes #14657 

An initial set of documentation for how our CI system works.

Documentation only, no production change.


[Here](https://github.com/cjllanwarne/hail/blob/cjl_document_ci/dev-docs/services/ci/README.md)'s
a link to a rendered version of the markdown.

Note to reviewers - please (***please***!) check the details to make
sure I got this right!