docs/migration: Create index page

Create an index page for migration module.  Move VFIO migration there too.
A trivial touch-up on the title to use lower case there.

Since then we'll have "migration" as the top title, make the main doc file
renamed to "migration framework".

Cc: Alex Williamson <alex.williamson@redhat.com>
Cc: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/r/20240109064628.595453-3-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

docs/devel/index-internals.rst

@@ -11,13 +11,12 @@ Details about QEMU's various subsystems including how to add features to them.
    block-coroutine-wrapper
    clocks
    ebpf_rss
-   migration/main
+   migration/index
    multi-process
    reset
    s390-cpu-topology
    s390-dasd-ipl
    tracing
-   vfio-migration
    vfio-iommufd
    writing-monitor-commands
    virtio-backends

docs/devel/migration/index.rst

@@ -0,0 +1,11 @@
+Migration
+=========
+
+This is the main entry for QEMU migration documentations.  It explains how
+QEMU live migration works.
+
+.. toctree::
+   :maxdepth: 2
+
+   main
+   vfio

docs/devel/migration/main.rst

@@ -1,6 +1,6 @@
-=========
-Migration
-=========
+===================
+Migration framework
+===================
 
 QEMU has code to load/save the state of the guest that it is running.
 These are two complementary operations.  Saving the state just does

docs/devel/migration/vfio.rst

@@ -1,5 +1,5 @@
 =====================
-VFIO device Migration
+VFIO device migration
 =====================
 
 Migration of virtual machine involves saving the state for each device that