Flag antora warnings [DOC-276] (#422)

We already need to build the documentation using Antora to check for
dead links - currently we log (and fail) if `errors` are introduced, but
we can drop to `warn`ings to further improve quality.

[Note that it's likely that the existing documentation content _already
has these problems_](hazelcast/hz-docs#1397), so
until addressed the exiting warnings are going to get flagged again and
again.

An [example of the
output](https://github.com/hazelcast/hz-docs/pull/1399/checks) can be
found in [this (dummy)
PR](hazelcast/hz-docs#1399).

This is an _alternative implementation_ of
hazelcast/hz-docs#1397

Fixes: [DOC-276](https://hazelcast.atlassian.net/browse/DOC-276),
hazelcast/hz-docs#1397



[DOC-276]:
https://hazelcast.atlassian.net/browse/DOC-276?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

.github/actions/validate/action.yml

@@ -28,4 +28,4 @@ runs:
         npm i
         npm i -D [email protected] [email protected]
         node load-check-links-playbook.js $GITHUB_REPOSITORY $GITHUB_BASE_REF
-        ./node_modules/.bin/antora --fetch --to-dir test --log-level=error --log-failure-level=error --extension=hazelcast-docs/lib/antora-link-checker-extension.js check-links-playbook.yml
+        ./node_modules/.bin/antora --fetch --to-dir test --log-level=warn --log-failure-level=warn --extension=hazelcast-docs/lib/antora-link-checker-extension.js check-links-playbook.yml