financials: add information about invoices and create an expenditure policy

The first 2 commits (invoices info) come as I've been asked a few times already. I do not want to have this information solely in my mind, so documenting it here, together with exemptions we have already use should be quite useful. If not to at least point to them next time somebody asks.

The expenditure policy is as discussed in https://wiki.postmarketos.org/wiki/Team_Meetings#2025-05-12. I've included not only small expenses by project managers, but also that the Board should not be able to approve expenses very much out of the budget. If controversial, I'm happy to remove it and keep discussing.

TODO:

• Link to this policy from OCE expenses policy