Begin renovating open source program handbook
This MR proposes several alterations to the handbook page outlining GitLab's open source programs. In particular, it:
- Reorganizes document headings to ensure consistency
- Specifies the sub-programs that comprise GitLab's open source initiative
- Updates the program's DRI (
bbehr
) - Relocates material on success measures to enhance readability
Additional revisions are forthcoming in additional merge requests.