doc/templates: add grey background and padding to code snippets
doc/templates: add grey background and padding to code snippets

Make the code snippets stand out a little better with a background and
some padding.

Change-Id: I0db487487f3fb22875cff6dba6458f5f57f0bd68
Reviewed-on: https://upspin-review.googlesource.com/11641
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

upspin/upspin,upspin/upspin