Document how to customize provider template files

[timoreimann]

A question we see fairly commonly is how to customize the template files. We should add a piece of documentation and make sure we cover the following topics:

• Outline reasons when one would use a customized template.
• Explain the structure of the template files (and the fact that it's just Go templates).
• Make clear that re-compilation of Traefik is not required.
• Mention the --filename parameters.
• Give an example.
• Possibly explain the data-flow related to templates very roughly (provider gets some kind of event update, populates the template data, renders the template, and sends it to the server component).