markdown: move to internal/markdown

This package was not made internal in CL 218517 because it was
intended to be used by packages in golang.org/x/blog. However,
golang.org/x/blog ended up taking a different approach that did
not include using golang.org/x/website/markdown; see CL 222846.

Since there isn't a need for this package to be non-internal,
make it internal while it still has 0 known public importers,
to keep future maintenance costs down.

For golang/go#33637.

Change-Id: Ie31264fe14b5cc68d98adbf62d6b824f1df579fb
Reviewed-on: https://go-review.googlesource.com/c/website/+/228418
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

content/static/internal/gen/gen.go

@@ -18,7 +18,7 @@ import (
 	"strings"
 	"unicode"
 
-	"golang.org/x/website/markdown"
+	"golang.org/x/website/internal/markdown"
 )
 
 var files = []string{

markdown/markdown.go → internal/markdown/markdown.go

@@ -3,8 +3,7 @@
 // license that can be found in the LICENSE file.
 
 // Package markdown provides a wrapper for rendering Markdown. It is intended
-// to be used by x/website and x/blog so that we can use the same renderer
-// with the same settings.
+// to be used on the golang.org website.
 //
 // This package is not intended for general use, and its API is not guaranteed
 // to be stable.