Documentation: Describe the site-local hook in our config/build system

Change-Id: Ia682b784540fa82e1f216f76d87d59a4f0b94486
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/51546
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Tim Wawrzynczak <twawrzynczak@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/tutorial/index.md

@@ -3,3 +3,4 @@
 * [Part 1: Starting from scratch](part1.md)
 * [Part 2: Submitting a patch to coreboot.org](part2.md)
 * [Part 3: Writing unit tests](part3.md)
+* [Managing local additions](managing_local_additions.md)