Add pointers to TinkerPop user group on Doc

TinkerPop has a user mailing list ([gremlin-users](https://groups.google.com/g/gremlin-users)) and a Discord server (https://discord.gg/ndMpKZcBEE). When users' questions are not JanusGraph-specific, we often redirect users to TinkerPop community channels. We could add pointers in JanusGraph README or even the website.

Review the JanusGraph README and website documentation entry points to determine the most appropriate place for community pointers. Use the two linked TinkerPop channels as the source material, and consider the work complete when users can find both channels with clear guidance about when to use them.

Written by the indexing model from the issue text.