content/static: fix css margins for playground examples
content/static: fix css margins for playground examples

The changes here will not appear until the display: none; experiment
flag is removed

If a playground example doesn't have an output and is accompanied by
another example right below, currently there is no margin that separates
the dropdown div and code input div above it. This change adds a
margin-top to the dropdown.

i.e: Non-executable example with no output
https://pkg.go.dev/github.com/prometheus/client_golang/prometheus?tab=doc#example-Timer
before: https://i.imgur.com/SpCNKQj.png
after:  https://i.imgur.com/iIr6ThE.png

If a playground example doesn't have an output, currently there is no
margin that separates the button and the code input div above it. This
change adds a margin-top on the button container.

i.e: Executable example with no output.
https://pkg.go.dev/cloud.google.com/go?tab=doc#example-package-ApplicationDefaultCredentials
before: https://i.imgur.com/yFMkKrt.png
after:  https://i.imgur.com/UkmqiHJ.png

The reason the margin-top in the button container doesn't add onto the
margin-bottom in an example with an output is because of margin
collapsing that occurs between margin top and bottom properties. This is
also true for the dropdown margin-top when the div above the dropdown is
not the code input div.

i.e: Executable example with output the before and after will remain the
same https://i.imgur.com/hSzjvBn.png

i.e: Dropdown with paragraph element above the before and after will
remain the same https://i.imgur.com/niyfG7B.png

For golang/go#36865

Change-Id: I5a271cb6cb899fcec6cbd22920e885c69c20b52b
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/247401
Reviewed-by: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

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