Docs enhancement request: document mkdocs-serve errors

[ljsinclair]

I haven't found any docs on mkdocs-serve errors I've come across and have solutions below for anyone who's interested:

Error	Cause	Example
PermissionError: [Errno 13] Permission denied	Leading / on URL in mkdocs.yml	/path/to/filename.md
IsADirectoryError: [Errno 21]	No trailing .md on filename in URL in mkdocs.yml	path/to/filename/
OSError [Errno 98]	Port 8080 assigned twice	* Run lsof -i :8000 to obtain the PID number nnnn for the port * Use the PID number in command: sudo kill 9 nnnn