Documentation/index.md: Add "Contributing" menu entry

Clean up the main menu by adding a new entry `Contributing` and moving
all related menu entries below it.

Change-Id: I04ec8a568b716df48ae7f8f826826e8753f5f88b
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/62220
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

0 files changed, 0 insertions, 0 deletions