Update the elements `User Guide` with instructions on how to use the `dj_local_conf`.

[kushalbakshi]

Since the workflow tutorials are now primarily based on CodeSpaces, the documentation should include detailed instructions on how to use dj_local_conf for the users setting up their workflow on a local machine.

This is based on comments on this PR.

This addition should be thorough enough to allow users to create a local DataJoint configuration and locally run a DataJoint workflow.