Write a basic Onboarding document

I'm trying out hiring people through Upwork to work on ActivityWatch, and I'd like to make the onboarding easier for new devs, so they can get up to speed quicker.

Should mention things like:

- About the project
- Mission/vision: https://github.com/ActivityWatch/activitywatch/issues/236
- Finding funding: https://github.com/ActivityWatch/activitywatch/issues/259
- [Getting started guide](https://docs.activitywatch.net/en/latest/getting-started.html)
- [Building from source guide](https://docs.activitywatch.net/en/latest/installing-from-source.html) (and other relevant materials in the docs)
- The Discord server
- [CONTRIBUTING.md](https://github.com/ActivityWatch/activitywatch/blob/master/CONTRIBUTING.md) file
- Enabling repository notifications
- Getting paid with ActivityWatch ([forum post](https://forum.activitywatch.net/t/getting-paid-with-activitywatch/986), [script](https://github.com/ActivityWatch/aw-client/blob/master/examples/working_hours.py))
- Where to pick work from
- [Project board](https://github.com/orgs/ActivityWatch/projects/2)
- Issue prioritization
- [Feature requests](https://forum.activitywatch.net/c/features/8/l/latest?order=votes)