Update track configuration file (config.json) documentation (#762)
Update track configuration file (config.json) documentation (#762)

* Add documentation for `ignore_pattern`
* Add what `test_pattern` is used for
* Change section heading so anchor will work.

Maybe it's possible to anchor to `config.json` but the period seemed to
be causing problems, so I renamed the section to be more linkable.

rpottsoh/x-common,ErikSchierboom/x-common,petertseng/x-common,rpottsoh/x-common,kgengler/x-common,jmluy/x-common,Vankog/problem-specifications,Vankog/problem-specifications,petertseng/x-common,kgengler/x-common,jmluy/x-common,ErikSchierboom/x-common,exercism/x-common,jmluy/x-common,exercism/x-common