Documentation: rename "Rookie guide" to "tutorial"

We generally try to stay away from ascribing attributes to (future)
devs. "Rookie guide" refers to the reader, while "tutorial" refers
to the material.

In the same spirit, move from "lessons" to "parts". It's not school :-)

Change-Id: I11a69a2a05ba9a0bc48f8bf62463d9585da043ec
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/35425
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Lance Zhao <lance.zhao@gmail.com>
Reviewed-by: Jacob Garber <jgarber1@ualberta.ca>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/tutorial/index.md

@@ -0,0 +1,4 @@
+# Tutorial
+
+* [Part 1: Starting from scratch](part1.md)
+* [Part 2: Submitting a patch to coreboot.org](part2.md)