Add Contributors section and Free Book giveaway page

bookshop_base_url param wires up the same shop.thistleandmoth.org
bookshop instance reground-infrastructure just provisioned
(giveaway-only for now, no Stripe account). Both pages are stubs --
Contributors has no content yet, and Free Book's link/title/code get
filled in once the real epub is uploaded and its ARC campaign created
(see reground-tools' arc-campaign.sh). The ARC redemption page itself
is fully self-hosted by the bookshop service, so no checkout/Stripe.js
template work is needed here for the giveaway to work.

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

content/contributors/_index.md

@@ -0,0 +1,6 @@
+---
+title: "Contributors"
+description: "Contributors published by Thistle & Moth."
+---
+
+Coming soon.

content/free-book/_index.md

@@ -0,0 +1,11 @@
+---
+title: "TODO: Free Book Title"
+description: "TODO: one- to two-sentence hook for the free book being given away."
+---
+TODO: free-book page copy goes here.
+
+This page is a stub — the link below won't resolve until the real ARC
+campaign is created (see reground-tools' `arc-campaign.sh`) and this copy
+is replaced with the real title/code.
+
+[Get your free ebook](https://shop.thistleandmoth.org/arc/TODO-real-code)