Update the get help section

What does this MR do and why?

This MR updates the Get Help section of the Contribute to GitLab development page to match recent updates to the section on the Contribute to GitLab page: https://about.gitlab.com/community/contribute/#get-help

These changes simplify the instructions for contributors to find help by guiding to our 3 most helpful sources.

Screenshot of the matching Get Help section from the Contribute to GitLab landing page:

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