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>

hugo.toml

@@ -13,12 +13,23 @@ disableKinds = ["taxonomy", "term"]
   description = "TODO: one-line description of Thistle & Moth"
   author = "Thistle & Moth"
   eec_base_url = "https://eec.thistleandmoth.org"
+  bookshop_base_url = "https://shop.thistleandmoth.org"
 
 [[menu.main]]
   name = "Submissions"
   url = "/submissions/"
   weight = 10
 
+[[menu.main]]
+  name = "Contributors"
+  url = "/contributors/"
+  weight = 20
+
+[[menu.main]]
+  name = "Free Book"
+  url = "/free-book/"
+  weight = 30
+
 [[menu.footer]]
   name = "Contact"
   url = "/contact/"