Documentation: Move Gerrit Guidelines to "Contributing" section

The Gerrit Guidelines are related to the contributing process and also
contain documentation which goes beyond "Getting started". Thus, move
them to the "Contributing" section.

Change-Id: I775a79c14562a1f4a9563012aee3b690c0635cc1
Signed-off-by: Felix Singer <felixsinger@posteo.net>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/62386
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/contributing/index.md

@@ -1,6 +1,7 @@
 # Contributing
 
 * [Coding Style](coding_style.md)
+* [Gerrit Guidelines](gerrit_guidelines.md)
 * [Project Ideas](project_ideas.md)
 * [Documentation Ideas](documentation_ideas.md)
 * [Google Summer of Code](gsoc.md)

Documentation/getting_started/index.md

@@ -4,7 +4,6 @@
 * [Build System](build_system.md)
 * [Submodules](submodules.md)
 * [Kconfig](kconfig.md)
-* [Gerrit Guidelines](gerrit_guidelines.md)
 * [Documentation License](license.md)
 * [Writing Documentation](writing_documentation.md)
 * [Setting up GPIOs](gpio.md)