doc: driver-model: Convert MIGRATION.txt to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/driver-model/index.rst b/doc/driver-model/index.rst
index 96fbd72..663aa5d 100644
--- a/doc/driver-model/index.rst
+++ b/doc/driver-model/index.rst
@@ -7,3 +7,4 @@
    :maxdepth: 2
 
    design
+   migration