Updated GitLab Onboarding Buddies with clearer guidance, ordered list, backup option

Updated onboarding-buddies based on feedback from corporate marketing new hire and discussions in PeopleOps Group Conversation on 2019-08-29

1. Changed bulleted (unordered) list to preferred numerical (ordered)
2. Per feedback from @monica_galletto, added "Encourage them to organize a group call with other new hires" for those who prefer a more social group new hire orientation experience (and linked to all-remote/learning-and-development)
3. Added clarity and suggestions on Slack channel recommendations to build a stronger sense of community early in the onboarding process
4. Added suggestion to ask them where they need help, directing them to experts on the GitLab team (e.g. new to a remote role, new to Git, new to Markdown, etc.), as discussed with @cteskey and the PeopleOps team on the 2019-08-29 Group Conversation
5. Added request to provide a backup buddy if original buddy will be out during critical first few weeks of onboarding.

Assigning to @bsexton due to this living in /handbook, but @cteskey you're welcome to provide feedback if desired.

FYI @melsmo @bchurch @jbandur

Thanks, all!