internal/frontend: prevent translation of markdown links in README files

Fixes golang/go#52683.

Change-Id: If34f46b2b3988b2fa1517ee304fa9c753b37aed2
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/406195
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
diff --git a/internal/frontend/overview.go b/internal/frontend/overview.go
index 4012f1c..13ceb61 100644
--- a/internal/frontend/overview.go
+++ b/internal/frontend/overview.go
@@ -142,6 +142,9 @@
 		if destURL.Host != "github.com" {
 			return ""
 		}
+		if strings.HasSuffix(destURL.Path, ".md") {
+			return ""
+		}
 		parts := strings.Split(destURL.Path, "/")
 		if len(parts) < 4 || parts[3] != "blob" {
 			return ""