docs: [WIP] Update routing/request/response docs

[provinzkraut]

This is an attempt to clean up the docs for routing, route handlers, parameters, requests and responses.

The individual chapters and sections currently lack clear boundaries and mix up a lot of topics, so that a user has to go through all of them to gather the necessary information.

The goal here is to split the sections not only functionality wise but also between usage and reference documentation, where the usage documentation should be more task focused (i.e. "how do I do X?"), while the reference should focus on the functionality of the components (i.e. "this class has these methods", "this decorator can be configured like so").

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This is a work in progress. I am currently mainly trying to establish an outline for a sensible structure.