Update CI to use rules instead of only/except, and remove unnecessary jobs

This MR updates the gitlab-ci configuration to use rules instead of only/except. This should hopefully make it more readable when jobs will/should run.

The only practical changes should be as follows:

• the render:{stage}:linux/osx jobs should now only run for MRs against the relevant {stage} (proposed or testing) OR when they are needed by the docs job triggered by changes to the docs/ source files. The full render job set (and the docs job) are still run for branch/tag builds on the main repo (computing/conda)
• the build:{stage} jobs should now only run for branch builds of the relevant branch (master for proposed and testing for testing) or for a tag build