create a demo budget for docs contribution

[tjex]

[...] maybe as an alternative you/we could make a demo budget and save it to this repo for testing purposes?

Originally posted by @j-f1 in #123 (review)

Including a small demo budget in the repo will help contributors to get going faster as they can simply import the demo budget and start working with the transactions, accounts, rules, etc that are already in place.

This will also create some better consistency throughout the documentation with accounts / transactions / payee naming etc.

I would imagine including the budget as a .zip backup export from Actual is the best way to deliver?