Documentation: Add several fixes

* Add support for Sphinx 3.0+
* Add backward support for Sphinx 1.8 and older
* Make sphinxcontrib ditaa an optional extension
* Allow SPHINXOPTS to be set from command line
* Add sphinx and sphinx-lint to top level Makefile

Change-Id: If10aef51dc426445cb742aad13b19ee7fe169c51
Signed-off-by: Patrick Rudolph <siro@das-labor.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41492
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

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