(DOCSP-35861): Add minimum disk capacity formula for Ops Manager Blockstore (#89)

Question for @MartinCanovas - what does `5%` refer to here? 5% of the
metadata size, 5% of the total calculation so far to allow for metadata,
or something else?

[Jira](https://jira.mongodb.org/browse/DOCSP-35861)

[Build](https://workerpool-boxgs.mongodbstitch.com/pages/job.html?collName=queue&jobId=65c68e4c6b2f17c95aa72366)

[Staging](https://preview-mongodbdavidhou17.gatsbyjs.io/ops-manager/DOCSP-35861/core/requirements/#backup-database-hardware-requirements)

Author: davidhou17

source/core/requirements.txt

@@ -249,13 +249,25 @@ If you store snapshots in the Backup Database, each data-bearing member
 must meet the following requirements:
 
 .. list-table::
-   :widths: 20 80
+   :widths: 20 40 40
    :header-rows: 1
 
    * - CPU Cores
+     - Disk Capacity
      - RAM
 
    * - 4 × 2 GHz+
+
+     - The minimum disk capacity required for the |onprem| blockstore
+       uses the following formula:
+
+       (2 to 3 times the total size of the ``dbPath``) * 2 (to allow 
+       :ref:`groom jobs <grooms-page>`)
+
+       Contact `MongoDB Support 
+       <https://support.mongodb.com/welcome?tck=docs>`__
+       to accurately determine your minimum disk capacity.
+
      - 8 GB of RAM per TB of blockstore disk to provide
        good snapshot and restore speed. |onprem| defines 1 TB of
        blockstore as 1024\ :sup:`4` bytes.