[doc] move all legacy host information into a dedicated section

It's just distracting to have legacy host application information
interleaved with the other stuff when it's deprecated.

doc/legacy_host.rst

@@ -0,0 +1,47 @@
+.. _legacy_host:
+
+Legacy Host Application
+#######################
+
+.. warning::
+
+   The Host Application is a legacy server application. New installations should
+   use the prebuilt :doc:`Looking Glass IDD <install_idd>`. These build
+   instructions are retained for the complete matching B7 stack described in
+   :ref:`legacy_host_policy`.
+
+.. note::
+
+   `Host` in this context refers to the `Looking Glass (LG) host server application`,
+   to which the LG client connects to, it thus does not refer to the `host OS`!
+
+   A common setup is to have a `host OS` running a `guest VM`. In such a setup
+   the `client application` runs on the `host OS`; and the `host application`
+   on the `guest VM`.
+
+   The term `Host application` was chosen over `Guest application`, because LG
+   can be run in a VM to VM configuration, in which case both Host Application and
+   Client Application are run inside VM Guests.
+
+The Windows Host Application is no longer the recommended server implementation.
+It is missing many of the features available in the Looking Glass IDD, such as:
+
+* No additional video capture overhead;
+* Usage without a monitor or dummy plug connected;
+* Automatically resizing the guest to fit the client application viewport;
+* HDR support; or
+* Usage in virtual machines without GPU acceleration.
+
+B7 is the last recommended version of the legacy host application for users who
+require its non-capture mouse input workflow. Use the complete matching B7 stack
+for that workflow rather than mixing B7 and current components.
+
+The documentation is included here for completeness.
+
+
+.. toctree::
+   :maxdepth: 1
+
+   Installation <install_host>
+   Configuration <host_usage>
+   Building from source <build_host>