x/pgksite: update frontend.md with JS mention
x/pgksite: update frontend.md with JS mention

Update doc to mention how to recompile JS

Change-Id: Ia0eab4471a6aba9484094d6bb9bb8c91e32109ac
GitHub-Last-Rev: 2d2c7c2ecaad43b38fc478902adb1ad5a69cf8f5
GitHub-Pull-Request: golang/pkgsite#17
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/269497
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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