renamed some classes and Twig functions to more descriptive names (refs #6871)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6871
| License       | MIT
| Doc PR        | symfony/symfony-docs#2205

Todo:

 - [x] update the docs

In #6871, @kriswallsmith wondered if the names used for the ESI/HIncludes sub-framework were meaningful enough.

I agree that this was not the case and I propose to remove the notion of **sub-requests** in favor of **fragments**. This sub-framework is a way to render fragments of a resource (the fact that it's done via another request is an implementation detail).

With that decision, all names can be renamed and are probably more meaningful. Some examples:

* `HttpContentRenderer` -> `FragmentHandler`
* `RouterProxyListener` -> `FragmentListener`
* `router_proxy` -> `fragments` (configuration entry)
* `DefaultRenderingStrategy` -> `InlineFragmentRenderer`