docs: bring 'reporting bugs page' into userguide and update it
We are trying to concentrate our documentation in read the docs and in our user/dev guides.
Let's make it more complete by also adding our 'Reporting bugs' page to it, since it's currently only in the redmine wiki (https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Reporting_Bugs), and readthedocs is the documentation we share with our community, these days.
Also update it to include a request to submit/create a suricata-verify test to go with the bug report, whenever possible. :)