Documentation: Explain FMAP and FMD

The Flashmap (FMAP) was not clearly documented. The new flashmap.md
explains where to find more details about that and how / why it was used
in coreboot. Also explained what is FMD and how to use it (based on
original README.fmaptool).

BUG=None
TEST=None (only documentation)

Change-Id: Ia389e56c632096d7c905ed221fd4f140dec382e6
Signed-off-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31766
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>

Documentation/lib/index.md

@@ -0,0 +1,7 @@
+# Library-specific documentation
+
+This section contains documentation about coreboot internal technical
+information and libraries.
+
+# Structure and layout
+- [Flashmap and Flashmap Descriptor](flashmap.md)