[DOC] Remove warnings from documentation build

[DOC] Remove warnings from documentation build

conf.py => use add_css_file as recommended
oom_developer_guide.rst => suppres anchor for nexus link

oom_user_guide.rst =>
- modify anchor for K8S link
- add `` fo http example to avoid link to be checked

For release-notes-<Release>.rst and oom_hardcoded_certificates.rst =>
- add :orphan: directive because rst file not in toctree

Issue-ID: OOM-2646

Signed-off-by: Eric Debeau <eric.debeau@orange.com>
Change-Id: Icfecde7807e2417a1825f25a1d57a1830c4222a0