Add troubleshoot docs for users that have conflicting files on existing install.

[kgilmer]

This issue is derived from #2 . Debian packages will fail to install if any files they attempt to write already exist on the target system. The existing Regolith docs assume a "clean" install of Ubuntu 18.04 which is probably not the case for most people, especially those that have used i3, rofi, compiz, etc. in the past.

Acceptance critera: users have a clear set of steps to move/remove conflicting files so that they can install Regolith without problems.

[kgilmer]

Added troubleshooting page with a basic description of package conflict problem. If this occurs more frequently may need to re-open and provide more specific steps to resolve.