Add /freebie and /freebie-thanks pages for the PDF lead magnet

/freebie reuses the courses/single layout (hero + signup form, same
mechanism as find-your-right-readers) via type: "courses" in front
matter, which controls layout lookup independently of the page's actual
URL — so the signup form is free, without the URL reading as
/courses/... for a single-PDF page. /freebie-thanks is a plain page
whose download link points at the new downloads.reground.org bucket.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

content/freebie-thanks/index.md

@@ -0,0 +1,8 @@
+---
+title: "Here's your PDF"
+description: "Download Three Steps to Find Your Right Readers."
+---
+
+Thanks for signing up — here's your copy of *Three Steps to Find Your Right Readers*.
+
+[Download the PDF](https://downloads.reground.org/three-steps-find-your-right-readers.pdf)