userguide: reorganize `Application Layers Parsers` and `Application layers` subsections in the suricata.yaml page
We currently have two subsections - `Application layers` and `Application Layer Parsers`
that could probably be merged into one.
After that is settled, fix some of the sub-section titles that currently have the wrong markdown,
causing them to be rendered as sub-subsections. (e.g. `SMB` - https://suricata.readthedocs.io/en/suricata-6.0.5/configuration/suricata-yaml.html#configure-smb).
The attached file shows the SMB example.