content: display documentation source files

A section for package files containing links to the source code
files was added in CL 240007.

These will now be displayed on pkg.go.dev.

For golang/go#37863

Change-Id: I35a7e74f631aa1f8c0a9775b0d721e7646843291
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/246437
Reviewed-by: Jonathan Amsterdam <[email protected]>

Author: julieqiu

content/static/css/stylesheet.css

@@ -1320,9 +1320,6 @@ code {
   margin-right: 0.4rem;
   padding-right: 0.5rem;
 }
-.Documentation-files {
-  display: none;
-}
 .Documentation-filesList {
   column-count: 3;
   column-width: 12.5rem;