Docs: Stop building in parallel processes

- Sphinx documentation rendering fails due to a missing image generated
  by PlantUML. Assuming this is caused by a race condition, this patch
  modifies the documentation build to run as a single process.

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I60875a743f2beb7507722030b2c2bc0bfa0f3a22
1 file changed