go.dev/_content: focus search input shortcut key
go.dev/_content: focus search input shortcut key

On non-firefox browsers the search input is focussed
after pressing the / key to match the behavior of
pkg.go.dev.

Fixes golang/go#42704

Change-Id: I81a804a9062f78997f3161ec86d1adcb8a911c92
Reviewed-on: https://go-review.googlesource.com/c/website/+/335669
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website