add link to implementation for each metric

[wholmgren]

Could link to...

the core function docstring: https://solarforecastarbiter-core.readthedocs.io/en/latest/generated/solarforecastarbiter.metrics.deterministic.normalized_mean_absolute.html#solarforecastarbiter.metrics.deterministic.normalized_mean_absolute

or to the RTD source: https://solarforecastarbiter-core.readthedocs.io/en/latest/_modules/solarforecastarbiter/metrics/deterministic.html#normalized_mean_absolute

I'm not aware of any way to cleanly link to the source on github based on function names.