DashboardBuilders: add notes on Python 3 version

As suggested at https://go.dev/issue/80220#issuecomment-5350598333,
it can be helpful to say a few more words about the exact Python 3
version that should be used. Also, as an optional reference, point
to a page where Python versions of existing builders can be looked
up.

For golang/go#80220.

Change-Id: I802b15a80fd348f51387a47e3e5093527171f9e5
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/818400
Reviewed-by: Eric Grosse <grosse@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>