add docs for nested dynamic disks

[DanRoscigno]

Closes ClickHouse/clickhouse-docs#970

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[DanRoscigno]

@kssenii I used type=url, I see from the tests that type=s3 is also supported. Are there others that work with dynamic?

[kssenii]

@kssenii I used type=url, I see from the tests that type=s3 is also supported. Are there others that work with dynamic?

yes, any disk type can be configured as dynamic, even Local disk, but dynamic local disks require path argument to be inside server config parameter custom_local_disks_base_directory, which has no default, e.g. will throw exception if local disk is created as dynamic disk and there is no such value in config.
And as an example of nested disk you can also use "encrypted" disk instead of "cache" disk.
You can also make 3-layer nested disk: encrypted over cached over web/s3/local.