Migrate existing Contributor Guide content into wiki-style format.

[m-miedema]

As we've discussed at length and especially as summarized here #49, we want to make Physiopy's contributor guide easier to follow and to update it.

Summary

First, we want to migrate the existing contributor guide into a wiki-style format that flows well with Physiopy's website while removing outdated information.

Next Steps

* Create an outline for the updated guide
* Migrate existing website pages relevant to the guide into new markdown pages according to said outline
* Eventually (in future issues) extend coverage of the guide and more thoroughly review its contents