[COMMON] Fix tox docs profiles configuration

[COMMON] Fix tox docs profiles configuration

- sphinx-build '-W' option needed only once
- remove docs/tox.ini since
  * same profiles are declared in tox.ini
  * once run from the docs folder, this generates invalid files
    in doc/.tox for linters setup (e.g.  docs/.tox/[..]/invalid.rst).
    They can cause warnings treated as errors by root tox profiles.
- modify requirements.txt to force a sphinxcontrib-needs version prior
  to 0.6.0 . This latest version introduces a new services support
  functionality that cannot be easily disabled here and that requires
  additional configuration parameters to link GitHub issues.
  We cannot provide these parameters since this project uses JIRA.
  As a result, sphinx generates a warning treated as an error
  that makes the gate fail.

https://sphinxcontrib-needs.readthedocs.io/en/service/services/index.html

Issue-ID: OOM-2648
Signed-off-by: Guillaume Lambert <guillaume.lambert@orange.com>
Change-Id: I14e9045482750f15a3e5e98af47197e2c9f60f60