docs: make oauth landing page more generic (#79)

* docs: make oauth landing page more generic

This is to support magics such as https://github.com/cloudspannerecosystem/spanner-graph-notebook

* add bqsql alias too

docs/source/oauth.rst

@@ -5,11 +5,11 @@
 
 .. _oauth-sign-in:
 
-Sign in to BigQuery
-===================
+Sign in to Google Cloud Platform
+================================
 
-You are seeing this page because you are attempting to access BigQuery via one 
-of several possible methods, including: 
+You are seeing this page because you are attempting to access Google Cloud Platform
+resources via one of several possible methods, including: 
 
   * the ``pydata-google-auth`` library
 
@@ -18,6 +18,11 @@ of several possible methods, including:
   * ``pandas.DataFrame.to_gbq()``
   * ``pandas.read_gbq()``
 
+  OR a Jupyter/IPython magics command such as:
+
+  * ``%%bigquery`` / ``%%bqsql``
+  * ``%%spanner_graph``
+
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