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/index.md

@@ -181,6 +181,7 @@ Contents:
 * [System on Chip-specific documentation](soc/index.md)
 * [Mainboard-specific documentation](mainboard/index.md)
 * [Payload-specific documentation](lib/payloads/index.md)
+* [Library-specific documentation](lib/index.md)
 * [SuperIO-specific documentation](superio/index.md)
 * [Vendorcode-specific documentation](vendorcode/index.md)
 * [Utilities](util.md)