This change allows the download page to redirect the user to
/doc/install?download=filename so the user can see installation
instructions specific to the file they are downloading.

This change also expands the "Test your Go installation" section
to instruct the user to create a workspace, hopefully leading
to less confusion down the line.

It also changes the front page download link to go directly
to the downloads page, which will in turn take them to the
installation instructions (the original destination).

This is related to this change to the tools repo:
https://golang.org/cl/13180

Change-Id: I658327bdb93ad228fb1846e389b281b15da91b1d
Reviewed-on: https://go-review.googlesource.com/13151Reviewed-by: Chris Broadfoot <cbro@golang.org>