document logging configuration and output files

https://books.sonatype.com/nexus-book/3.0/reference/install.html#directories

This section of the book makes no reference to where a user can find the Nexus log files, and what the log files contain.

This makes a poor user experience if a user is trying to debug a startup problem in particular, because they do not know where to look for logs or what the different log files could contain.

Acceptance:

• document log *configuration* file locations that are visible on disk
  • what are they used for internally
  • when should they be manually edited, if at all
  • possibly do not document at all files that should never be edited manually ( see NEXUS-9304 )
• document log output files that are actual logs containing output of the application, located in the $data-dir/log directory
  • jvm.log
  • nexus.log
  • karaf.log
  • request.log
  • archives of nexus and request log

Probably out of scope:

• document how to customize log rotation settings, including location for output
• document console commands like nexus:logger