Ramdisk deploy driver doc

Docs for the ramdisk driver, at least at a basic level.

Change-Id: Ie5f829fa6789f8a3e09eb1c9454f79b4315ae95a
Task: #23164
Story: #1753842

doc/source/admin/interfaces/deploy.rst

@@ -91,3 +91,55 @@ it, see :doc:`Ansible deploy interface <../drivers/ansible>`.
    :hidden:
 
    ../drivers/ansible
+
+Ramdisk deploy
+==============
+
+The ramdisk interface is intended to provide a mechanism to "deploy" an
+instance where the item to be deployed is in reality a ramdisk.
+Most commonly this is peformed when an instance is booted via PXE or iPXE,
+with the only local storage contents being those in memory. Initially this
+is only supported by the ``pxe`` boot interface, but other boot interfaces
+could support this funtionality in the future.
+
+As with most non-default interfaces, it must be enabled and set for a node
+to be utilized:
+
+.. code-block:: ini
+
+   [DEFAULT]
+   ...
+   enabled_deploy_interfaces = iscsi,direct,ramdisk
+   ...
+
+Once enabled and the conductor(s) have been restarted, the interface can
+be set upon creation of a new node or update a pre-existing node:
+
+.. code-block:: shell
+
+   openstack baremetal node create --driver ipmi \
+       --deploy-interface ramdisk \
+       --boot-interface pxe
+   openstack baremetal node set <NODE> --deploy-interface ramdisk
+
+The intended use case is for advanced scientific and ephemeral workloads
+where the step of writing an image to the local storage is not required
+or desired. As such, this interface does come with several caveats:
+
+* Configuration drives are not supported.
+* Disk image contents are not written to the bare metal node.
+* Users and Operators who intend to leverage this interface should
+  expect to leverage a metadata service, custom ramdisk images, or the
+  ``instance_info/ramdisk_kernel_arguments`` parameter to add options to
+  the kernel boot command line.
+* Bare metal nodes must continue to have network access to PXE and iPXE
+  network resources. This is contrary to most tenant networking enabled
+  configurations where this access is restricted to the provisioning and
+  cleaning networks
+* As with all deployment interfaces, automatic cleaning of the node will
+  still occur with the contents of any local storage being wiped between
+  deployments.
+
+.. warning::
+   As of the Rocky release of the BareMetal service, only the ``pxe`` boot
+   interface is supported.