Documentation: Update index.md and move files

* Add more subdirectories and index.mds.
* Move "getting started" and "lessons" into sub-directories.
* Move "NativeRaminit" into northbridge/intel/sandybridge folder.
* Move "MultiProcessorInit" into soc/intel/icelake folder.
* Reference new files

Change-Id: I78c3ec0e8bcc342686277ae141a88d0486680978
Signed-off-by: Philipp Deppenwiese <zaolin@das-labor.org>
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/26262
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/index.md

@@ -1,5 +1,4 @@
-Welcome to coreboot's documentation!
-====================================
+# Welcome to the coreboot documentation
 
 This is the developer documentation for [coreboot](https://coreboot.org).
 It is built from Markdown files in the
@@ -8,16 +7,14 @@ directory in the source code.
 
 Contents:
 
-* [Lesson 2: Submitting a patch to coreboot.org](Lesson2.md)
-* [Gerrit Etiquette and Guidelines](gerrit_guidelines.md)
-* [coreboot's build system](build_system.md)
-* [Kconfig in coreboot](core/Kconfig.md)
-* [Use of git submodules in coreboot](submodules.md)
+* [Getting Started](getting_started/index.md)
+* [Rookie Guide](lessons/index.md)
 * [Timestamps](timestamp.md)
 * [Dealing with Untrusted Input in SMM](technotes/2017-02-dealing-with-untrusted-input-in-smm.md)
 * [ABI data consumption](abi-data-consumption.md)
 * [GPIO toggling in ACPI AML](acpi/gpio.md)
 * [Native Graphics Initialization with libgfxinit](gfx/libgfxinit.md)
-* [Sandy Bridge Raminit](Intel/NativeRaminit/Sandybridge.md)
+* [Northbridge-specific documentation](northbridge/index.md)
+* [System on Chip-specific documentation](soc/index.md)
 * [Mainboard-specific documentation](mainboard/index.md)
 * [SuperIO-specific documentation](superio/index.md)