Add documentation on how to run migrations using pg-diff

[jpmckinney]

https://michaelsogos.github.io/pg-diff/

• Make changes from issue title to https://pelican-backend.readthedocs.io/en/latest/tasks/database.html
• Update the deploy repository's documentation and/or states https://ocdsdeploy.readthedocs.io/en/latest/deploy/data-support.html#pelican-backend
• Add a paragraph on migrations to the ORM term in the handbook (use built-in features with Django, alembic with SQLAlchemy, and pg-diff with plain SQL, linking here)

This issue can wait until we need to edit the SQL files.

WIP

{
    "development": {
        "sourceClient": {
            "host": "localhost",
            "port": 5432,
            "database": "pelican_backend_dev",
            "applicationName": "pg-diff-cli"
        },
        "targetClient": {
            "host": "localhost",
            "port": 5432,
            "database": "pelican_backend",
            "applicationName": "pg-diff-cli"
        },
        "compareOptions": {
            "outputDirectory": "sqlscripts",
            "getAuthorFromGit": true,
            "schemaCompare": {
                "namespaces": "public",
                "dropMissingTable": false,
                "dropMissingView": false,
                "dropMissingFunction": false,
                "dropMissingAggregate": false,
                "roles": []
            },
            "dataCompare": {
                "enable": false
            }   
        },
        "migrationOptions": {
            "patchesDirectory": "pelican/migrations",
            "historyTableName": "migrations",
            "historyTableSchema": "public"
        }        
    }
}