content/static/css: make .JumpDialog's position fixed.
content/static/css: make .JumpDialog's position fixed.

add `position:fixed;` to `.JumpDialog` class to always keep it on visible viewport even if the page is scrolled.
fixes golang/go#43840

Change-Id: Id692319fff66cd26453acbf50000da92e9f6e373
GitHub-Last-Rev: 47879a522fbb601d6b8cc382f45e11b9e5615c8e
GitHub-Pull-Request: golang/pkgsite#21
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/285674
Trust: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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