Break root module docs into separate files (#432)

For 0.3 I'm thinking about some changes I wanna make to the docs. I
don't like how information is currently spread over so many places.
Still thinking about how I wanna re-organize it.

However I do think it makes sense to break the root module docs into
separate files that get included with `#![doc = include_str!("file")]`.
Makes working on a single section easier and more focused. It looks the
same for the user reading the docs.

This means axum's MSRV is now 1.54 but since thats two releases ago I'm
fine with that.

src/docs/table_of_contents.md

@@ -0,0 +1,29 @@
+# Table of contents
+
+- [High level features](#high-level-features)
+- [Compatibility](#compatibility)
+- [Handlers](#handlers)
+    - [Debugging handler type errors](#debugging-handler-type-errors)
+- [Routing](#routing)
+    - [Wildcard routes](#wildcard-routes)
+    - [Nesting routes](#nesting-routes)
+    - [Fallback routes](#fallback-routes)
+    - [Routing to any `Service`](#routing-to-any-service)
+        - [Routing to fallible services](#routing-to-fallible-services)
+- [Extractors](#extractors)
+    - [Common extractors](#common-extractors)
+    - [Applying multiple extractors](#applying-multiple-extractors)
+    - [Optional extractors](#optional-extractors)
+    - [Customizing extractor responses](#customizing-extractor-responses)
+- [Building responses](#building-responses)
+- [Error handling](#error-handling)
+- [Applying middleware](#applying-middleware)
+    - [To individual handlers](#to-individual-handlers)
+    - [To groups of routes](#to-groups-of-routes)
+    - [Applying multiple middleware](#applying-multiple-middleware)
+    - [Commonly used middleware](#commonly-used-middleware)
+    - [Writing your own middleware](#writing-your-own-middleware)
+- [Sharing state with handlers](#sharing-state-with-handlers)
+- [Required dependencies](#required-dependencies)
+- [Examples](#examples)
+- [Feature flags](#feature-flags)