content/static: add margin to bottom of last element in nav group
content/static: add margin to bottom of last element in nav group

Fixes issue where a single item in a nav group
like Index > Examples looks cluttered.

Change-Id: Icf9103c4f6e7b57339df7187ca593f4ef604367f
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/260727
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite