Enable all Sphinx build warnings as errors

It also fixes several issues to pass the gate.
Please note that .. code-block :: json content must be json.

Issue-ID: DOC-798
Signed-off-by: Cédric Ollivier <cedric.ollivier@orange.com>
Change-Id: I38a751b67ab70a5ed5d61cdd49fe914c7b9270cd
diff --git a/docs/tox.ini b/docs/tox.ini
index ab2de1b..8684276 100644
--- a/docs/tox.ini
+++ b/docs/tox.ini
@@ -27,7 +27,7 @@
     -chttps://raw.githubusercontent.com/openstack/requirements/stable/yoga/upper-constraints.txt
     -chttps://git.onap.org/doc/plain/etc/upper-constraints.onap.txt?h=master
 commands =
-    sphinx-build -q -b html -n -d {envtmpdir}/doctrees {toxinidir} {toxinidir}/_build/html
+    sphinx-build -W -q -b html -n -d {envtmpdir}/doctrees {toxinidir} {toxinidir}/_build/html
 [testenv:docs-linkcheck]
 basepython = python3.8
 deps =
@@ -35,7 +35,7 @@
     -chttps://raw.githubusercontent.com/openstack/requirements/stable/yoga/upper-constraints.txt
     -chttps://git.onap.org/doc/plain/etc/upper-constraints.onap.txt?h=master
 commands =
-    sphinx-build -b linkcheck -d {envtmpdir}/doctrees {toxinidir} {toxinidir}/_build/linkcheck
+    sphinx-build -W -q -b linkcheck -d {envtmpdir}/doctrees {toxinidir} {toxinidir}/_build/linkcheck
 [testenv:docs-spellcheck]
 basepython = python3.8
 deps =
@@ -43,4 +43,4 @@
     -chttps://raw.githubusercontent.com/openstack/requirements/stable/yoga/upper-constraints.txt
     -chttps://git.onap.org/doc/plain/etc/upper-constraints.onap.txt?h=master
 commands =
-    sphinx-build -b spelling -Dextensions=sphinxcontrib.spelling -d {envtmpdir}/doctrees {toxinidir} {toxinidir}/_build/spellcheck
+    sphinx-build -b spelling -d {envtmpdir}/doctrees {toxinidir} {toxinidir}/_build/spellcheck