Update online documentation, remove outdated docs (#323)

- Removed outdated developer and admin documentation.
- Moved documents such as the Cheatsheet or the OAuth user guide into the
  online docs, which are now the 'single source of truth' for users.
- Extended the Cheatsheet to cover sending data with multipart/form-data
- Added general introductions to REST and API auth.
- In the online docs, added support for:
  - Query parameters.
  - Authorization header.
  - Additional lookup path for GET.
  - Error Responses.
- Hooks are not displayed anymore, as they are only relevant for API developers
  and not for users.
- Updates ReDoc to version 2.

Regarding the documentation updates, I briefly considered putting them into
a pull request to Eve-Swagger. However, development on Eve-Swagger seems stale,
with two open pull requests without response. Furthermore, most additions
include AMIV API specific detail (e.g. a link to the cheatsheet), so I opted
for a API specific solution instead.