docs: Move microvm.rst into the system manual

Now that target-i386.rst has a place to list documentation of
machines other than the 'pc' machine, we have a place we can
move the microvm documentation to.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

docs/microvm.rst → docs/system/i386/microvm.rst

@@ -1,6 +1,5 @@
-====================
-microvm Machine Type
-====================
+'microvm' virtual platform (``microvm``)
+========================================
 
 ``microvm`` is a machine type inspired by ``Firecracker`` and
 constructed after its machine model.

docs/system/target-i386.rst

@@ -16,6 +16,7 @@ Board-specific documentation
 .. toctree::
    :maxdepth: 1
 
+   i386/microvm
    i386/pc
 
 .. include:: cpu-models-x86.rst.inc