Add a page which lists and describes all Bro logs

Add a page to documentation listing the Bro log files.

There's already a page listing common log files:
http://www.bro.org/sphinx/logs/index.html#common-log-files

This page could be moved to http://www.bro.org/sphinx/script-reference/index.html and be expanded to list all known log files.

If this goes well, Robin suggested adding a btest that heuristically checks if something changes with regards to which logs we have.