Add more explicit instructions for adding user and compute identities, setting Scope(s) on each

[dotsdl]

Currently, it is left as an exercise to the reader in step 7 of the deployment instructions to navigate to the main alchemiscale docs and determine how to add both a user and compute identity and set Scope(s) for these.

Instead, we should add explicit, easy-to-follow instructions here for how to do this in practice for at least one user and one compute identity, and one non-specific Scope.