Revamp the `Filters` page
In this PR we:
- add a section about the filtering process, that explains in detail all the places,
where a filter can be configured.
- add the description of 3 filters,
- improve the documentation of the remaining filters, by replacing the complete XML configuration examples with shorter examples in 4 formats that concentrate on the filter itself.
Closes #2529,
[LOG4J2-3485](https://issues.apache.org/jira/browse/LOG4J2-3485)
and
[LOG4J2-3682](https://issues.apache.org/jira/browse/LOG4J2-3682).
42 files changed
