Create a dedicated onboarding for Issue Triage specialists

Issue Triage specialists currently don't have a dedicated onboarding.

I think this is needed because their job is between support:

• Issue Triage specialists have to perform support on GitLab CE/EE and GitLab.com Support Tracker
• Issue Triage specialists have to have access to production/staging console to be able to gather data if needed

and development:

• Issue Triage specialists are expected to fix community-reported bugs

but also has its own particularities:

• Issue Triage specialists have to know the available labels (for relevant labelling)
• Issue Triage specialists have to know the Engineering team members and their speciality (for relevant pings)

We could start simple by stealing some information / documents from other onboardings (e.g. https://about.gitlab.com/handbook/support/onboarding/checklist/) and adding checklist for the following items:

• https://about.gitlab.com/handbook/engineering/issues/issue-triage-policies/
• https://about.gitlab.com/handbook/developer-onboarding/ (maybe not everything)
• https://about.gitlab.com/handbook/engineering/workflow/