Department responsibilites in the handbook

### Description

There's uncertainty what _exactly_ the responsibility is per department. This creates uncertainty in the organisation on who creates what decision. There's a [DRI section in the handbook](https://flowfuse.com/handbook/company/decisions/#directly-responsible-individual), which however isn't always applied. While work on that, let's also work on the following:

> Every department lists their responsibilities in the index for the departments handbook page. And who's ultimately responsible for reaching that goal

- [x] @knolleary for engineering
- [x] @MarianRaphael for product
- [ ] @iskerrett for marketing
- [x] @zackwasli for sales
- [ ] @joepavitt for UX/Design

### Epic/Story

_No response_

### Have you provided an initial effort estimate for this issue?

I have provided an initial effort estimate