DOCSP-32634: Add Indexes page (#206)

Author: mcmorisi

source/fundamentals.txt

@@ -25,6 +25,7 @@ Fundamentals
    /fundamentals/specify-query
    /fundamentals/serialization
    /fundamentals/transactions
+   /fundamentals/indexes
    /fundamentals/logging
    /fundamentals/time-series
    /fundamentals/encrypt-fields
@@ -43,6 +44,7 @@ Fundamentals
 - :ref:`csharp-specify-query`
 - :ref:`csharp-serialization`
 - :ref:`csharp-transactions`
+- :ref:`csharp-indexes`
 - :ref:`csharp-logging`
 - :ref:`csharp-time-series`
 - :ref:`Encrypt Fields <csharp-fle>`