(DOCSP-8369) Explain what indexes are on the Manage Indexes page (#212)

* WIP

* (DOCSP-8369) Overview of indexes

* Formatting and word choice

* Changes based on review

* Further changes

* Changes based on another round of review

* Finalizing changes

* Limitations to bottom of page

Author: zach-carr

source/indexes.txt

@@ -12,6 +12,32 @@ Manage Indexes
    :depth: 1
    :class: singlecol
 
+Indexes are special data structures that improve query performance.
+Indexes store a portion of a collection's data in an easy-to-traverse 
+form. The index stores the value of a specific field or set of fields, 
+ordered by the value of the field.
+
+To improve query performance, build indexes on fields that appear often 
+in queries and for all operations that 
+:ref:`sort by a field <query-bar-sort>`.
+
+- Queries on an indexed field can use the index to limit the number of 
+  documents that must be scanned to find matching documents.
+
+- Sort operations on an indexed field can return documents pre-sorted 
+  by the index.
+
+To learn more about indexes, see :manual:`Indexes </indexes>`.
+
+.. admonition:: Considerations
+   :class: note
+
+   Indexes have some negative performance impact on write operations. 
+   For collections with high write-to-read ratio, indexes are expensive 
+   since each insert must also update any indexes. For a detailed list 
+   of considerations for indexes, see
+   :manual:`Operational Considerations for Indexes <core/data-model-operations/#data-model-indexes>`.
+
 .. _indexes-tab:
 
 Indexes Tab
@@ -47,14 +73,6 @@ For each index, Compass displays the following information:
      - Any special properties (such as uniqueness, partial) of the
        index.
 
-Limitations
------------
-
-- .. include:: /includes/extracts/readonly-not-permitted-indexes.rst
-
-- The :guilabel:`Indexes` tab is not available if you are connected 
-  to a :atlas:`Data Lake </data-lake>`. 
-
 .. _create-index:
 
 Create an Index
@@ -71,3 +89,11 @@ Drop an Index
 ---------------
 
 .. include:: /includes/steps/drop-index.rst
+
+Limitations
+-----------
+
+- .. include:: /includes/extracts/readonly-not-permitted-indexes.rst
+
+- The :guilabel:`Indexes` tab is not available if you are connected 
+  to a :atlas:`Data Lake </data-lake>`.