Add test for navigation SCHEMA validation

Building on the work of @eread in #1158 (comment 1303288299), and using the spec that @mgandres provided in #1158 (comment 1303807509), I created a job to test for the navigation.yaml schema.

https://github.com/davishmcclurg/json_schemer is mentioned in https://json-schema.org/implementations.html#validator-ruby.

• Failed test with a duplicate doc_title entry and an invalid - doc_url entry https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/3899020925
• Successful test https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/3899052456

The only drawback is that the failed job is not descriptive at all as to where the error is.

Closes #1158 (closed).