[OOM] mutualize sphinx linting

[OOM] mutualize sphinx linting

docs and docs-linkcheck are both using sphinx
and can run easily in parallel without interfering.
Both jobs take less than 3 minutes.
This is often less than the allocation time of a jenkins minion.
All of this make the mutualization of these 2 jobs wishable.

- use one subproject for docs and docs-linkcheck tox profile.
- enable tox parallel mode

Issue-ID: OOM-2648
Signed-off-by: guillaume.lambert <guillaume.lambert@orange.com>
Change-Id: I2f61fa489ca7ab543a667d07009bf32929329456