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.