Package level documentation for go.dev

[Southclaws]

https://pkg.go.dev/github.com/picostack/pico?tab=doc

It doesn't need to be all-encompassing, just links out to the Pico project page etc.

[Southclaws]

This will mostly get done along with the overal documentation effort for the tool and the wider stack. Docs will live in github.com/picostack/picostack