Revamp file and delegating appenders

This PR splits the documentation of file appenders and the `Async`, `Failover`, `Rewrite` and `Routing` appenders into their own files.

As in the previous PRs we:

* check and reformat the configuration options,
* add links to the Plugin Reference,
* improve the readability of the description by shortening it and reformatting it,
* remove examples to provide smaller ones that make the same point.

Part of #2528.
53 files changed