Document that Metabase needs super user on the installation on Azure (CREATE EXTENSION Error)

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**Is your feature request related to a problem? Please describe.**
When a new Metabase instance is launched, the migration script automatically attempts to run commands like the following:
```
CREATE EXTENSION IF NOT EXISTS citext WITH SCHEMA public
CREATE EXTENSION IF NOT EXISTS uuid-ossp WITH SCHEMA public
```

However, in Azure Database for PostgreSQL, admin privileges are required to execute CREATE EXTENSION. This means that even if the extensions are already installed, the migration still attempts to run these commands, resulting in a permissions error.

**Describe the solution you'd like**
The request is for Metabase to first check whether an extension exists before attempting to create it. This would eliminate the need for workarounds in automated deployments.

**Describe alternatives you've considered**
Currently, the workaround involves “tricking” the migration by restoring a backup of the databasechangelog table from another instance to bypass the extension creation step.

Start by locating the migration script that runs CREATE EXTENSION for citext and uuid-ossp, then review how the databasechangelog table records that migration. Verify the behavior against Azure Database for PostgreSQL when the extensions already exist, and confirm that a new Metabase instance completes migration without a permissions error.

Written by the indexing model from the issue text.