doc: tweak front page playground

[adg]

a) link to the tour from the playground interface
b) make it clearer that 'share' is effectively the new 'pop out'

[bradfitz]

Comment 1:

I noticed the new playground doesn't have the fun drop-down of examples. Intentional?

[adg]

Comment 2:

A sin of omission. Consider that within the scope fo this bug, too.

[adg]

Comment 3:

This issue was closed by revision d528f52.

Status changed to Fixed.