How-to Docs: Implemeting an exercise from problem spec

[junedev]

A contributor from the Go track noticed that in the legacy docs, there was this page https://exercism.org/docs/building/tracks/new/add-initial-exercises#h-implement-exercises but it does not have a good equivalent in the V3 docs.

I found parts of it here https://exercism.org/docs/building/tracks/new/add-initial-exercises#h-implement-exercises but this was "hidden" under the creating a new track section. I don't think a regular contributor to an existing track would look there.

There is also some info in the problem spec readme I think.

In any case, I think it would be good to bring it all together and describe how to implement an exercise from the problem spec repo in a track on one docs page.