A bookkeeping documentation section (WIP)

[e8johan]

This is a WIP pull request - do not merge!

I'm working on a chapter on the bookkeeping features of the system. There are quite a few outstanding items left. The following list is incomplete (feel free to add more):

• Account names are in Swedish
• Account setup for all payment methods is needed
• Chapter on how to create invoices is empty
• Chapter on how to track payments is empty
• Chapter on report generation is empty
• Info on when to use a reverse signs for an account is missing
• Info on when to make an account available for invoicing is missing
• Info on what accounts to "fold" is missing

[mhagander]

First of all -- +many for writing these docs, that'll be very useful!

But, in most of the rest of the system I believe this is referred to as "accounting". This is a pretty ad-hoc term for it in the beginning, but unless there is a substantial reason to use bookkeeping instead I think we should keep that term (even if it's slightly incorrect). And if it actually isn't correct, then perhaps we also need to adjust the existing terminology?