simpler doc tasks

this adds some tasks under a new `:docs` namespace to make regenerating docs a little faster so the editing process can be more efficient. And it auto-launches the browser for convenience.

**before:**

0. edit a doc comment
0. `rake generate:docs` (wait ~65 sec)
0. refresh the browser

**after:**

0. edit a doc comment
0. `rake docs:refresh` (wait ~55 sec)
0. _no step 3!_