Documentation: ACPI: move scan_handlers.txt to driver-api/acpi and convert to reST

This converts the plain text documentation to reStructuredText format
and adds it to Sphinx TOC tree.

No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/driver-api/acpi/index.rst

@@ -6,3 +6,4 @@ ACPI Support
    :maxdepth: 2
 
    linuxized-acpica
+   scan_handlers