New chapters: delete, patch, errors, whats-next

Hiya!

Phew, finally, the last chapters of episode 1 are written! This PR includes a few other things (like deleting some old notes files), but here is what's up for review for technical/explanation accuracy (there will may be typos, I'll do another pass late to fix this!):


* [ ] A) delete
* [ ] B) patch
* [ ] C) errors
* [ ] D) application-problem
* [ ] E) error-consistency
* [ ] F) error-invalid-json
* [ ] G) error-api-problem-exception
* [ ] H) error-404-json
* [ ] I) error-details
* [ ] J) whats-next

Thanks!