Documentation: move `coding_style.md` inside contributing/

Keep less files in the root directory.

Change-Id: I9eebd0b0826181340ead41af5284362d1cca09d7
Signed-off-by: Alexey Vazhnov <vazhnov@boot-keys.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/50852
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Michał Żygowski <michal.zygowski@3mdeb.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
diff --git a/Documentation/coding_style.md b/Documentation/contributing/coding_style.md
similarity index 100%
rename from Documentation/coding_style.md
rename to Documentation/contributing/coding_style.md
diff --git a/Documentation/index.md b/Documentation/index.md
index 82ff12d..9870cf3 100644
--- a/Documentation/index.md
+++ b/Documentation/index.md
@@ -162,7 +162,7 @@
 
 * [Getting Started](getting_started/index.md)
 * [Tutorial](tutorial/index.md)
-* [Coding Style](coding_style.md)
+* [Coding Style](contributing/coding_style.md)
 * [Project Ideas](contributing/project_ideas.md)
 * [Documentation Ideas](contributing/documentation_ideas.md)
 * [Code of Conduct](community/code_of_conduct.md)