Update the Getting Help page

https://about.gitlab.com/getting-help/

Changes

Update advice for finding help for GitLab.com

• Already links to the correct tracker
• Just a reword

Change the Technical Support section

• Split the CE and EE tech support options
• Add a link to the documentation website
• Mention the forum and StackOverflow
• Move the mailing list to discussion section

Change the discussion section

• Add feature proposals and bug reports for issue trackers here
• Add advice regarding the posting of issues to trackers
• Move the discussion section above tech. support section
• Remove advice to add labels, only team members can now label issues
• Update links to relevant parts of the contributing guide

Related Issues

Resolves #762 (closed)

cc @stanhu @balameb @dblessing @haynes