[COMMON] Remove parallelism in doc linting

`sphinxcontrib.needs` is not safe for parallel work and thus a warning
is set during linting which makes it fail.
We then remove the parallelism for sphinx here.

Issue-ID: OOM-2648
Signed-off-by: Sylvain Desbureaux <sylvain.desbureaux@orange.com>
Change-Id: I820e63fee78578794c96c3ff9867c4ecc56ba50e
1 file changed