docs: Missing `databricks` backend docs

[filipeo2-mck]

Please describe the issue

Hello!

First: thanks for this amazing project :)

I noted that databricks is now enabled as a new backend (#10223, it's also listed under Backend support matrix), but I see no documentation on how to make the connection properly.

Digging into the source code, I noted

• the need for DATABRICKS_SERVER_HOSTNAME, DATABRICKS_HTTP_PATH and DATABRICKS_TOKEN env vars
• the .connect() needs additional arguments:

  con = ibis.databricks.connect(
      catalog="main",
      schema="default",
  )

• it relies on Unity Catalog Volumes. At some point of the connection, it tries to create a Volume which is available only in the premium tiers (the need for Unity Catalog). When using a non-premium, I got:

  ServerOperationError: [[UC_VOLUMES_NOT_ENABLED](https://docs.microsoft.com/azure/databricks/error-messages/error-classes#uc_volumes_not_enabled)] Support for Unity Catalog Volumes is not enabled on this instance. SQLSTATE: 56038
  == SQL ==
  CREATE VOLUME IF NOT EXISTS `root-py=31012-pid=2012` COMMENT 'Ibis memtable storage volume'

It would be awesome if those (and additional) minimal requirements were declared under the "Backends" section of the docs.

I've tested it using ibis-framework==10.2.0, Python 3.10.

Thank you.

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