[website] Make it clear that communication channels (inc gitlab issues) are internal only

• slack, mailing list and gitlab issues are internal only. Make this clear.

Thank about a channel for external users to stay up to date or contribute.

• update CONTRIBUTING.md with a route for external users

• Add "WIN member" and "External Users" buttons to contact page
• Consider adding "(internal only)" to "request a feature" link.

marked the checklist item slack, mailing list and gitlab issues are internal only. Make this clear. as completed

marked the checklist item update CONTRIBUTING.md with a route for external users as completed

mentioned in commit 2c1d3144

closed