Add link to GitHub discussion forum to "Getting help" documentation

[ptitzler]

This PR adds a link to the new discussion forum to the Getting help topic in the Elyra documentation at https://elyra.readthedocs.io/en/latest/getting_started/getting-help.html. This provides users with another venue to ask questions, hopefully reducing the need to create an issue.

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