udpate: add more detail to github doc

[mathetos]

We need to make more explicit how we use Github as Support Technicians, specifically how we use Zenhub to communicate the progress of tickets and how that informs how we communicate with Customers.

This is the existing doc: https://give.gitbook.io/support-manual/daily-routine/contributing-to-github-issues

These are some of my first thoughts on what is missing from there currently: https://docs.google.com/document/d/1gUaykCvksOJ_gKNOuye2FP0YiivtyYPknNvkv4XEKqk/edit#