Expand docs with batching example

Add a short doc on the use of `BatchSQLAlchemyConnectionField` with a brief comment. It's only documented in the test files, but should also be included in the documentation.

Additionally, can expand docs on `sort` keyword to include disabling and custom usage.

_Originally posted by @palisadoes in https://github.com/graphql-python/graphene-sqlalchemy/issues/357#issuecomment-1447481010_