(DOCSP-23341): Add system.buckets to system collections page (#1307)

* (DOCSP-23341): Add system.buckets to system collections page

* wording

* ordering

* add 'do not modify'

* updates per review

* add link

* updates per review feedback

Author: jeff-allen-mongo

source/reference/system-collections.txt

@@ -13,7 +13,7 @@ System Collections
 .. contents:: On this page
    :local:
    :backlinks: none
-   :depth: 1
+   :depth: 2
    :class: singlecol
 
 Synopsis
@@ -64,9 +64,21 @@ database:
    The :data:`~config.system.indexBuilds` collection stores 
    information related to in-progress index builds.
 
+Database-Specific Collections
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
 System collections also include these collections stored directly in
 each database:
 
+.. data:: <database>.system.buckets
+
+   Stores the underlying data associated to a :ref:`time series
+   collection <manual-timeseries-collection>`. The ``system.buckets``
+   collection stores data in an optimized format and schema for an
+   efficient representation of the persisted time series data.
+
+   Do not modify this collection.
+
 .. data:: <database>.system.namespaces
 
    .. note:: Removed in 4.2