static/shared: update footer with link to pkg.go.dev/about
static/shared: update footer with link to pkg.go.dev/about

Video:https://drive.google.com/file/d/10dfZQgpEh2590taRXw6lYd0pcbhvg6u1/view?usp=sharing

Change-Id: Ifc6323b9f5fc1560bd0cccc99a44b3fe0e0ba189
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/443115
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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