Update docs for top-level API. (#13592)

This PR adds documentation for `cudf.from_dataframe`, `cudf.date_range`, and `cudf.interval_range`. These are exposed in the top-level API of the `cudf` module but lacked docstrings.

Authors:
  - Bradley Dice (https://github.com/bdice)

Approvers:
  - GALI PREM SAGAR (https://github.com/galipremsagar)

URL: #13592

docs/cudf/source/api_docs/general_functions.rst

@@ -26,13 +26,23 @@ Top-level conversions
    :toctree: api/
 
     cudf.to_numeric
+    cudf.from_dataframe
     cudf.from_dlpack
     cudf.from_pandas
 
-Top-level dealing with datetimelike
------------------------------------
+Top-level dealing with datetimelike data
+----------------------------------------
 
 .. autosummary::
    :toctree: api/
 
     cudf.to_datetime
+    cudf.date_range
+
+Top-level dealing with Interval data
+------------------------------------
+
+.. autosummary::
+   :toctree: api/
+
+    cudf.interval_range