Add docs for ci services shared builds directory

What does this MR do and why?

I was looking all over to find this documented. It seems there is a passing mention of this earlier on this page related to healthchecks:

The service container is not providing any networking service, but it’s doing something with the job’s directory (all services have the job directory mounted as a volume under /builds). In that case, the service does its job, and because the job is not trying to connect to it, it does not fail.

But when looking around I was expecting to find a specific mention of a shared filesystem or something as it seems an important detail for what is possible. In my case I was slightly surprised when I introduced a service that was dependent on having access to these same files and it "just worked". I later started scratching my head thinking "how does this work if they're different containers?".

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.