Improve troubleshooting documentation for the Job failed: prepare environment error with the Shell executor (docs)

What does this MR do?

Adds a more specific troubleshooting step to the shell profile loading documentation page for the following error:

ERROR: Job failed: prepare environment: exit status 1. Check https://docs.gitlab.com/runner/shells/index.html#shell-profile-loading for more information

Why was this MR needed?

As noted in this community forum thread, the current troubleshooting steps are not very helpful in providing users a quick solution to the error for a newly configured Runner on Linux with the Shell executor,.