documentation: Add documentation ideas for season of docs

Let's gather some documentation ideas for the season of docs. I reused
the project ideas style (thanks Patrick). Feel free to add yourself as a
mentor here. Also if you have more ideas, please add them to the
document.

Change-Id: I72221cbd53b99cdc946109753cf72af9c865a1e5
Signed-off-by: Christian Walter <christian.walter@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/40662
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/index.md

@@ -164,6 +164,7 @@ Contents:
 * [Tutorial](tutorial/index.md)
 * [Coding Style](coding_style.md)
 * [Project Ideas](contributing/project_ideas.md)
+* [Documentation Ideas](contributing/documentation_ideas.md)
 * [Code of Conduct](community/code_of_conduct.md)
 * [Community forums](community/forums.md)
 * [Project services](community/services.md)