docs: updates keycloakx readme about creating the admin user

[wilmardo]

Since #621 was merged this doesn't reference anything:

Please refer to the section on database configuration for how to configure a secret for this.

Also the environment variables look wrong to me (edit: not only me #605)? From the docs:

use the KEYCLOAK_ADMIN and KEYCLOAK_ADMIN_PASSWORD environment variables to create an initial admin account.

-- https://www.keycloak.org/docs/latest/server_admin/#creating-the-account-remotely

So here my attribution to the docs, feedback is of course welcome :)

[github-actions[bot]]

This pull request has been marked as stale because it has been open for 30 days with no activity. It will be automatically closed in 10 days if no further activity occurs.

[wilmardo]

Not really stale?

[github-actions[bot]]

This pull request has been marked as stale because it has been open for 30 days with no activity. It will be automatically closed in 10 days if no further activity occurs.

[wilmardo]

Not stale, this should be an easy review PR :)

[grieshaber]

Thanks @wilmardo and sorry for the extremlly slow review.