fix: add updating password step to GeoNode Basic Installation guide

[brynsofz]

This PR updates the GeoNode Basic Installation documentation by adding a missing step for changing a secure password when needed

Change Introduced

• Added a section describing how to update the GeoNode and GeoServer admin passwords during the installation.
• Added a section describing how to generate and configure new OAuth2 credentials.

[giohappy]

Thanks for the PR @brynsofz.

• Why "Fourth Step"? I don't see the Third.... If you consider the section about the .env customisation, the third step, I would update it too.
• The step shouldn't be required only for manual setups, since the Docker-based guide invites you to generate the .env file using the create-envfile.py script, which assigns random passwords for admins. In this case, we should prompt the user to double check that this step was applied.