Describe tab conditionalization

Add details of how to repeat tabbed content conditionally for builds that do not support
tabs.

Change-Id: I2201a4f29a300ab501deccaecfae5dd209424d3b
Signed-off-by: Ron Stone <ronald.stone@windriver.com>

doc/source/contributor/doc_contribute_guide.rst

@@ -785,6 +785,77 @@ Try it out for yourself here:
 
       Windows tab content - tab set 3
 
+***********************************
+Considerations for alternate builds
+***********************************
+
+It is important to make allowances for builds of StarlingX documentation in
+contexts that may not support tabs, such as by other parties or for print
+output. To support such environments, the use of tabs must be conditionalized
+and the related content repeated for non tab scenarios. The following sample
+illustrates the standard way to achieve this:
+
+.. code-block::
+
+   .. only:: starlingx and html
+
+   .. tabs::
+
+      .. group-tab:: Bare Metal
+
+         .. begin-install-prereqs-bm
+
+         .. include:: /shared/_includes/installation-prereqs.rest
+            :start-after: begin-install-prereqs
+            :end-before: end-install-prereqs
+
+         .. end-install-prereqs-bm
+
+      .. group-tab:: Virtual
+
+         .. begin-install-prereqs-virt
+
+         Several pre-requisites must be completed prior to starting the |prod|
+         installation.
+
+         Before attempting to install |prod|, ensure that you have the the
+         |prod| host installer ISO image file.
+
+         Get the latest |prod| ISO from the `StarlingX mirror
+         <https://mirror.starlingx.cengn.ca/mirror/starlingx/release/latest_release/debian/monolithic/outputs/iso/>`__.
+         Alternately, you can get an older release ISO from `here <https://mirror.starlingx.cengn.ca/mirror/starlingx/release/>`__.
+
+         .. end-install-prereqs-virt
+
+
+   .. only:: partner and (html or latex)
+
+      **********
+      Bare Metal
+      **********
+   
+      .. include:: /deploy_install_guides/release/bare_metal/aio_duplex_install_kubernetes.rst
+         :start-after: begin-install-prereqs-bm
+         :end-before: end-install-prereqs-bm
+
+      *******
+      Virtual
+      *******
+
+      .. include:: /deploy_install_guides/release/bare_metal/aio_duplex_install_kubernetes.rst
+         :start-after: begin-install-prereqs-virt
+         :end-before: end-install-prereqs-virt
+
+In this example, the tabbed content is conditionalized to only appear in
+StarlingX HTML output (``.. only:: starlingx and html``).
+   
+The ``.. begin-install-prereqs-bm``, ``.. end-install-prereqs-bm``,  ``..
+begin-install-prereqs-virt`` and ``.. end-install-prereqs-virt`` comments allow
+the contents of the ``Bare Netal`` and ``Virtual`` tabs to be included
+conditionally in other builds that do not support tabs. In this example the
+content is conditionalized in the ``.. only:: partner and (html or latex)``
+block.
+
 .. end-using-tabbed-content
 
 ------------------------
@@ -1183,9 +1254,9 @@ Results in:
 
 2. Red
 
-1. Blue
+3. Blue
 
-2. Green
+4. Green
 
 To avoid this, use substitute .. pre\ |html-comment|-include:: for ``.. include::``