doc/start: s/intro.rst/index.rst/

Change the filename "doc/start/intro.rst" to "doc/start/index.rst" so
that Sphinx finds the root filename for the "/start" directory in the
default location.

Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 84ce2212e87a4b6b2416eeab7e8e1718ae3ce87b)

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    :maxdepth: 3
    :hidden:
 
-   start/intro
+   start/index
    install/index
    cephadm/index
    rados/index

diff --git a/doc/start/index.rst b/doc/start/index.rst
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+===============
+ Intro to Ceph
+===============
+
+Ceph can be used to provide :term:`Ceph Object Storage` to :term:`Cloud
+Platforms` and Ceph can be used to provide :term:`Ceph Block Device` services
+to :term:`Cloud Platforms`. Ceph can be used to deploy a :term:`Ceph File
+System`.  All :term:`Ceph Storage Cluster` deployments begin with setting up
+each :term:`Ceph Node` and then setting up the network. 
+
+A Ceph Storage Cluster requires the following: at least one Ceph Monitor and at
+least one Ceph Manager, and at least as many :term:`Ceph Object Storage
+Daemon<Ceph OSD>`\s (OSDs) as there are copies of a given object stored in the
+Ceph cluster (for example, if three copies of a given object are stored in the
+Ceph cluster, then at least three OSDs must exist in that Ceph cluster).
+
+The Ceph Metadata Server is necessary to run Ceph File System clients.
+
+.. note::
+
+   It is a best practice to have a Ceph Manager for each Monitor, but it is not
+   necessary. 
+
+.. ditaa::
+
+            +---------------+ +------------+ +------------+ +---------------+
+            |      OSDs     | | Monitors   | |  Managers  | |      MDSs     |
+            +---------------+ +------------+ +------------+ +---------------+
+
+- **Monitors**: A :term:`Ceph Monitor` (``ceph-mon``) maintains maps of the
+  cluster state, including the :ref:`monitor map<display-mon-map>`, manager
+  map, the OSD map, the MDS map, and the CRUSH map.  These maps are critical
+  cluster state required for Ceph daemons to coordinate with each other.
+  Monitors are also responsible for managing authentication between daemons and
+  clients.  At least three monitors are normally required for redundancy and
+  high availability.
+
+- **Managers**: A :term:`Ceph Manager` daemon (``ceph-mgr``) is
+  responsible for keeping track of runtime metrics and the current
+  state of the Ceph cluster, including storage utilization, current
+  performance metrics, and system load.  The Ceph Manager daemons also
+  host python-based modules to manage and expose Ceph cluster
+  information, including a web-based :ref:`mgr-dashboard` and
+  `REST API`_.  At least two managers are normally required for high
+  availability.
+
+- **Ceph OSDs**: An Object Storage Daemon (:term:`Ceph OSD`,
+  ``ceph-osd``) stores data, handles data replication, recovery,
+  rebalancing, and provides some monitoring information to Ceph
+  Monitors and Managers by checking other Ceph OSD Daemons for a
+  heartbeat. At least three Ceph OSDs are normally required for 
+  redundancy and high availability.
+
+- **MDSs**: A :term:`Ceph Metadata Server` (MDS, ``ceph-mds``) stores metadata
+  for the :term:`Ceph File System`. Ceph Metadata Servers allow CephFS users to
+  run basic commands (like ``ls``, ``find``, etc.) without placing a burden on
+  the Ceph Storage Cluster.
+
+Ceph stores data as objects within logical storage pools. Using the
+:term:`CRUSH` algorithm, Ceph calculates which placement group (PG) should
+contain the object, and which OSD should store the placement group.  The
+CRUSH algorithm enables the Ceph Storage Cluster to scale, rebalance, and
+recover dynamically.
+
+.. _REST API: ../../mgr/restful
+
+.. container:: columns-2
+
+   .. container:: column
+
+      .. raw:: html
+
+          <h3>Recommendations</h3>
+
+      To begin using Ceph in production, you should review our hardware
+      recommendations and operating system recommendations.
+
+      .. toctree::
+         :maxdepth: 2
+
+         Beginner's Guide <beginners-guide>
+         Hardware Recommendations <hardware-recommendations>
+         OS Recommendations <os-recommendations>
+
+   .. container:: column
+
+      .. raw:: html
+
+          <h3>Get Involved</h3>
+
+      You can avail yourself of help or contribute documentation, source
+      code or bugs by getting involved in the Ceph community.
+
+      .. toctree::
+         :maxdepth: 2
+
+         get-involved
+         documenting-ceph
+
+.. toctree::
+   :maxdepth: 2
+
+   intro