Docs/cbfstool: Add details about memory mapped window handling

This change adds details memory mapped window handling in cbfstool
required for x86 platforms. It also captures the details about the
newly added support for multiple decode windows.

BUG=b:171534504

Change-Id: Icf970f951e56d717e6a4f8845fc73f10d5a21dd0
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48477
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Reviewed-by: Duncan Laurie <dlaurie@chromium.org>

Documentation/cbfstool/index.md

@@ -0,0 +1,5 @@
+# cbfstool
+
+Contents:
+
+* [Handling memory mapped boot media](mmap_windows.md)