have http server in-line PDF files.

add go course pdf files to doc/ for godoc to serve.
fix links to point to those files.

R=rsc
DELTA=7  (4 added, 0 deleted, 3 changed)
OCL=35972
CL=35981

src/pkg/http/fs.go

@@ -22,6 +22,7 @@ var contentByExt = map[string] string {
 	".html":	"text/html; charset=utf-8",
 	".jpg":	"image/jpeg",
 	".js":	"application/x-javascript",
+	".pdf":	"application/pdf",
 	".png":	"image/png",
 }