internal/godoc/dochtml: only show examples in sidenav if present
internal/godoc/dochtml: only show examples in sidenav if present

This change fixes a bug where the examples section was always shown as a
subsection of Index in the sidenav. Examples is now only shown if that
section exists.

Change-Id: Ic08fca93dedba302a6f53b3cf9dcf01107c8a0fa
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/260197
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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