docs: Move deprecation, build and license info out of system/

Now that we have a single Sphinx manual rather than multiple manuals,
we can provide a better place for "common to all of QEMU" information
like the deprecation notices, build platforms, license information,
which we currently have in the system/ manual even though it applies
to all of QEMU.

Create a new directory about/ on the same level as system/, user/,
etc, and move these documents there.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210705095547.15790-5-peter.maydell@linaro.org

docs/about/index.rst

@@ -0,0 +1,10 @@
+About QEMU
+==========
+
+.. toctree::
+   :maxdepth: 2
+
+   build-platforms
+   deprecated
+   removed-features
+   license

docs/index.rst

@@ -10,6 +10,7 @@ Welcome to QEMU's documentation!
    :maxdepth: 2
    :caption: Contents:
 
+   about/index
    system/index
    user/index
    tools/index

docs/system/index.rst

@@ -35,7 +35,3 @@ or Hypervisor.Framework.
    targets
    security
    multi-process
-   deprecated
-   removed-features
-   build-platforms
-   license