update

this will be a pretty major update. ~~hope to get it done by friday.~~

## Todo

- [x] finish remaining updates (just a couple pages)
- [x] make sure there are no unreferenced images
- [x] write payments article/section
- [x] signatures article
see screenshots in commit comment: https://github.com/dobtco/knowledge_base/commit/7eb6e4c6357dbd7a266575be3f7b393fc58d313e