DOC: filters: Add filters documentation

The configuration documention has been updated. Doc about the filter line has
been added and a new chapter (ยง. 9) has been created to list and document
supported filters (for now, flt_trace and flt_http_comp).

The developer documentation about filters has also been added. The is a "pre"
version. Incoming changes in the filter API will require an update.
This documentation requires a deeper review and some TODO need to be complete.
2 files changed