doc: update frontend docs
doc: update frontend docs

Adds a section to the frontend docs that details
the new frontend development process.

For golang/go#40960

Change-Id: Ia9d89c05486819a6a236af7d69a5416f03257ab5
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/281776
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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