[RFC] Clean up kernel by introducing ParamConverters

Right now, we use magic in `Kernel.php` to define the contenttype routes like `/pages`, `/pages/4`, etc. It is a hefty operation that also parses yaml files, it happens here:

https://github.com/bolt/core/blob/master/src/Kernel.php#L124-L142

But this logic can also be implemented with ParamConverters, which will be "the Symfony way" of achieving dynamic parameters in routes:
https://symfony.com/bundles/SensioFrameworkExtraBundle/current/annotations/converters.html

Start with Kernel.php lines 124-142, where the current content type route logic and YAML parsing are implemented. Read the linked Symfony ParamConverters documentation and compare it with the existing route behavior for /pages and /pages/4. Done means the dynamic route handling is implemented with ParamConverters instead of the current Kernel magic, without changing the supported routes.

Written by the indexing model from the issue text.