Restructured docs

This removes the descriptive table of contents that nobody reads anyway.

To do:

* [ ] Fix URL structure to match parts in documentation - **delayed until later**
* [x] Set up redirects for old URLs
  * `/docs/reference/commands/:name/`
  * `/docs/reference/filters/:name/`
  * `/docs/reference/helpers/:name/`
  * `/docs/reference/`
  * `/docs/guides/`