docs: restructure documentation

This rewrite follows the principles of https://diataxis.fr/

Co-authored-by: Erik Michelson <github@erik.michelson.eu>
Signed-off-by: Philip Molares <philip.molares@udo.edu>
Signed-off-by: Erik Michelson <github@erik.michelson.eu>

docs/content/dev/dev_notes.md

@@ -1,32 +0,0 @@
-# Development Notes
-
-This document collects notes and decisions taken during the development of HedgeDoc 2.0.
-It should be converted to a properly structured documentation, but having unstructured docs
-is better than having no docs.
-
-## Supported databases
-
-We intend to officially support and test these databases:
-
-- SQLite (for development and smaller instances)
-- PostgreSQL
-- MariaDB
-
-## Special Groups
-
-The software provides two special groups which have no explicit users:
-
-- `everyone` (Describing that everyone who wants to access a note can do if it is
-   enabled in the config.)
-- `loggedIn` (Describing all users which are logged in)
-
-## Entity `create` methods
-
-Because we need to have empty constructors in our entity classes for TypeORM to work, the actual
-constructor is a separate `create` method. These methods should adhere to these guidelines:
-
-- Only require the non-optional properties of the corresponding entity
-- Have no optional parameters
-- Have no lists which can be empty (so probably most of them)
-- Should either return a complete and fully useable instance or return a Pick/Omit type.
-- Exceptions to these rules are allowed, if they are mentioned in the method documentation