[Docs]: How to retrieve credentials for Supervisor interface

[jnikhila]

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Documentation Link

https://docs.appsmith.com/getting-started/setup/instance-management/supervisor

Discord/slack/intercom Link

https://theappsmith.slack.com/archives/C0341RERY4R/p1693282690960659?thread_ts=1692895315.977639&cid=C0341RERY4R

Describe the problem and improvement.

Update the doc to retrieve the supervisor credentials.

[sum35h]

I'm not sure if we need doc's specific to retrieving the supervisor ui credentials, but they are always available as env variables APPSMITH_SUPERVISOR_USER and APPSMITH_SUPERVISOR_PASSWORD which gets specified in the docker.env file or other env manifests for other platforms like the values.yml in case of Helm.

A user can always update the above 2 env vars with the new credentials and restart the container for the updated values to take effect.