Documentation updates on multitenancy

[rathorevaibhav]

Summary

1. Installation steps updated on creating a public tenant. This is a required part of setup otherwise the package results in error.

Test Plan

Nothing required.

[dalmia]

I think this will be good after 2 minor changes: - either changing `psycopg2` to `psycopg2-binary` and checking if we still get the error / fixing it some other way - Adding the `createdb` command after starting postgres service in case the error that Deepansh highlighted shows up.

[dalmia]

@rathorevaibhav I went ahead and made the changes - let me know if it's fine!

[rathorevaibhav]

@dalmia Did some testing and psycopg-binary is also working fine for me. This looks good!