Add shell executor concurrency caching example

What does this MR do?

Adds a new cache configuration example which avoids issues when using the shell executor and where build file paths vary on subsequent job runs due to the inclusion of the CI_CONCURRENT_PROJECT_ID in the path to allow for the running of concurrent jobs.

For example, the first run of job A may use a build path with CI_CONCURRENT_PROJECT_ID=0:

/home/gitlab-runner/builds/<runner_id>/0/path/to/project

and another run of the same job may use a build path with CI_CONCURRENT_PROJECT_ID=1 if there is already another job running:

/home/gitlab-runner/builds/<runner_id>/1/path/to/project

If references to the build paths are included in the cache then build errors may result if the cached path and current path don't match.

Also update the description of the CI_CONCURRENT_PROJECT_ID variable to indicate it forms part of shared build paths.

This scenario and the example solution were reported by a customer in (ZD internal link)

Related issues

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