Documentation: Convert abi-data-consumption.txt to Markdown

This will help static site generators that can turn Markdown into HTML
but can't easily turn plain text into HTML.

Change-Id: Id186db140503f3c2759d579b18476fff021988c8
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/25559
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

Documentation/abi-data-consumption.md

@@ -1,9 +1,11 @@
+# ABI data consumption
+
 This text describes the ABI coreboot presents to downstream users. Such
 users are payloads and/or operating systems. Therefore, this text serves
 at what can be relied on for downstream consumption. Anything not explicitly
 listed as consumable is subject to change without notice.
 
-Background and Usage
+## Background and Usage
 
 coreboot passes information to downstream users using coreboot tables. These
 table definitions can be found in src/include/boot/coreboot_tables.h and