[STAR-2719] Reorganize budgeting/sustainability doc and add section about billable/non-billable work

This MR does three things:

• billable_work.md adds a new section in our handbook explaining how Billable vs. Non-Billable / Support vs. Maintenance work works, so that developers better understand the implications of each type of account (and their impact on billing and sustainability).
• it removes the documentation related to budgeting/sustainability from epic_planning_and_sustainability_management.md , and instead creates a dedicated section for it (sustainability_management.md) for improved readability and discoverability. [<-this is out of the scope of STAR-2719, but I got the idea while doing it]
• The section describing the types of billable accounts is moved from sustainability_management.md to billable_work.md