Doc - adds info about user to be used to start the starter

[sleto-it]

Now ready for review / merging

[Simran-B]

Changed formatting and style a bit, improved cross-reference labels

[sleto-it]

Adding Kaveh for a quick review from the Cluster Team.

Kaveh, in the PR there are a few different changes (you can basically ignore them I believe), but the main point of this PR is the following:

We are suggesting to not run the arangodb using root, but using a limited privileged user, so the command we are suggesting is this one (assuming the user is arangodb:

sudo su arangodb -s /bin/bash -c "arangodb --all.server.uid=$(id -u arangodb) ... AnyStarterOptionHere"

When possible, please let us know if you believe this approach is okay, or if you have any feedback. As soon as this is merged, this will be incorporated into the official documentation and also Support will start suggesting this approach

If other approaches are needed - please let us know

Thanks!!

[nikita-vanyasin]

@Simran-B can you please confirm if this PR is still needed? I think all actual documentation for starter was moved to main docs web-site long time ago.