x/pkgsite: make import paths in Examples into links

[Sajmani]

What is the URL of the page with the issue?

https://pkg.go.dev/gonum.org/v1/gonum/graph/encoding/dot

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/90.0.4430.93 Safari/537.36

Screenshot

What did you do?

I clicked "gonum.org/v1/gonum/graph/simple"

What did you expect to see?

Browser opens https://pkg.go.dev/gonum.org/v1/gonum/graph/simple

What did you see instead?

Nothing. It's plain text. 😞

[smasher164]

The playground widget would need to be updated for this.
One tricky bit here is that <textarea> elements don't let you
put clickable links in them, so the widget would have to be
rewritten to support it.