V4 docs: Remove example trailing slash middleware since it is not compatible

[cookieguru]

The example middleware to remove the trailing slash from routes in the Slim V4 documentation is not PSR-15 compatible, and thus does not work in Slim 4. Thankfully, the linked middleweare is, so update the docs to point solely to that middleware.

Additionally, clarify that rewriting a request of a non-GET operation only affects the in-memory copy of the Request object, and not the actual user called by the consumer.

[akrabat]

Rebased against latest gh-pages.

[akrabat]

Thanks!