internal/web: stop double-escaping of code in non-playground examples

4c9e549253 introduced an issue causing non-playground
code examples to be double-escaped, causing HTML escape sequences and elements
to appear in the code, such as the examples in crypto/rsa, as reported in golang/go#46839.
This change treats the code as `template.HTML` rather than `string` after running
through `(*Page).Node` (which formats the code as HTML) and doing some basic transformations,
preventing this double-escaping from occurring.

Fixes golang/go#46839

internal/web/examplefuncs.go

@@ -74,9 +74,10 @@ func (p *Page) Example(funcName string) template.HTML {
 
 		newPage := *p
 		newPage.Data = struct {
-			Name, Doc, Code, Play, Output string
+			Name, Doc, Play, Output string
+			Code                    template.HTML
 		}{
-			eg.Name, eg.Doc, code, play, out,
+			eg.Name, eg.Doc, play, out, template.HTML(code),
 		}
 		err := t.Execute(&buf, &newPage)
 		if err != nil {