Extend documentation on ARM crossbuild runner and how to add custom ARM runners

The README has a section on how to activate ARM builds: https://salsa.debian.org/salsa-ci-team/pipeline#build-jobs-on-arm

However, there is no section on how to create runners. Gitlab does have extensive docs on how to add your custom runners, but these have several kind of executors and a lot of options to choose. It would be much more straightforward if the configuration/setup of a reference runner somebody is already using for the 'SALSA_CI_DISABLE_BUILD_PACKAGE_ARM*` options.

Related, one of my packages (MariaDB) is rather large, and the test-crossbuild-arm64 job does not complete in the 1 hour apparently configured in the runner itself, and thus timeouts. This is probably intentional from the maintainers of the runner, and runner timeouts can't be overridden by project or pipeline settings. Thus I'd like to use use a custom runner, but it feels overwhelming to create one and fiddling with the setup on a trial-and-error manner, so being able to read the docs of the current runners on howto setup one yourself would be very useful.

One more thing to consider here is that potential contributors forking a project on Salsa and using CI to validate their changes before finalizing a Merge Request might have failing pipelines as they don't have access to the custom runners a project might have. Thus as part of this documentation I would also like to see what are the recommended strategies for automatically skipping certain CI jobs if a runner with a specific tag is not available.

If somebody gives enough pointers as comments in this issue I volunteer to write (or to polish) the documentation on custom runners for use with Salsa-CI based pipelines.