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/soc/intel/index.md

@@ -0,0 +1,7 @@
+# Intel SOC-specific documentation
+
+This section contains documentation about coreboot on specific Intel SOCs.
+
+## Platforms
+
+- [Ice Lake/9th Gen Core-i series](icelake/index.md)