Document Duo for JetBrains TLS certificate configuration

Problem to solve

Our GitLab Duo for JetBrains IDEs do not document how users should connect to server's with self-signed certificates or certificates not in the default JetBrains IDE certificate store.

Proposal

Add a new section documenting the required settings to connect to a server without a default/full certificate chain.

Who can address the issue

@erran @bohdanpk @jgdoyon1 @aqualls

Other links/references

1. https://www.jetbrains.com/help/idea/settings-tools-server-certificates.html
2. https://www.jetbrains.com/help/idea/ssl-certificates.html