gitlab: default to not building the documentation

gitlab: default to not building the documentation

In d0f26e68a0 ("gitlab: force enable docs build in Fedora, Ubuntu,
Debian") we made sure we can build the documents on more than one
system. However we don't want to build documents all the time as it's
a waste of cycles (and energy). So lets reduce the total amount of
documentation we build while still keeping coverage of at least one
build on each supported target.

Fixes: a8a3abe0b3 ("gitlab: move docs and tools build across from Travis")
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210323165308.15244-23-alex.bennee@linaro.org>