DOC: filters: Document how to port an existing filter

DOC: filters: Document how to port an existing filter

The filters guide now contains a section about the conversion of a legacy
filter to the filter instance mode. It lists the changes from a filter
developer point of view, then explains the minimal changes needed to support
both declaration modes, how to fully use the instance mode, and the pitfalls
and design considerations to keep in mind.

In particular, it details the callback timing and the parsing-time
constraints, the ownership of the filter configuration and of its fields,
the enabled state and its consequences, the class registration and
placement, the legacy mode coexistence, and the lifecycle of the filters
of a proxy created at runtime.