docs: Fail out on documentation warnings
docs: Fail out on documentation warnings

Previously, documentation warnings were ignored with regards to the
success of the `test-docs` target, which lead to developers introducing
a variety of warnings into the documentation about broken links and so
on. Fix this by ensuring that the stderr is redirected to a file and
validating that the file is empty after running the basic sphinx
spellcheck. Only if there's no warnings (other than the known ones we
can ignore), let the target succeed. Otherwise print a helpful message
asking the developer to fix the warnings.

Signed-off-by: Joe Stringer <16a9a54ddf4259952e3c118c763138e83693d7fd@cilium.io>

cilium-team/cilium,michi-covalent/cilium,tgraf/cilium,cilium/cilium,tgraf/cilium,cilium/cilium,michi-covalent/cilium,tgraf/cilium,tgraf/cilium,tgraf/cilium,tklauser/cilium,tgraf/cilium,cilium/cilium,tklauser/cilium,cilium/cilium,cilium-team/cilium,tklauser/cilium,michi-covalent/cilium,cilium/cilium,michi-covalent/cilium,tklauser/cilium,tklauser/cilium,michi-covalent/cilium