doc: add link to google cloud deploy doc
doc: add link to google cloud deploy doc

Change-Id: I38ffcfba7915f4ef4e4dada82f7f72700e87ed8d
Reviewed-on: https://upspin-review.googlesource.com/7918
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@google.com>

upspin/upspin,upspin/upspin