doc: Convert build documentation to sphinx

These build instructions are mostly based on `Documentation/building.md`
and the wiki.

There are some  `.. todo::`  sections still present in the
documentation. They will be completed later. For some of the todos
content needs to be written, some others require custom sphinx-plugins
to be implemented.
The `.. todo::` sections are only visible in the source, not in the
rendered html.

Change-Id: I96771e98b313a6d26dd2be940ff37998d4124324
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/73359
Reviewed-by: Alexander Goncharov <chat@joursoir.net>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

doc/dev_guide/index.rst

@@ -0,0 +1,7 @@
+Developers documentation
+========================
+
+.. toctree::
+    :maxdepth: 1
+
+    building_from_source