Using docker to build to documentation eases the process of building the documentation. Given that some versions of sphinx are incompatible, the option to use docker is presented first. Change-Id: I6c18f81a829364ada1859c04ba2dc4f886934bcc Signed-off-by: Arthur Heymans <arthur@aheymans.xyz> Reviewed-on: https://review.coreboot.org/c/coreboot/+/36105 Reviewed-by: Patrick Georgi <pgeorgi@google.com> Tested-by: build bot (Jenkins) <no-reply@coreboot.org>