Add welcome course stub (day-0 only)

active: false until day-1+ copy is ready. Entered via /subscribe's
SUBSCRIBE_ENROLL_COURSE_SLUG=welcome, not signup_list_name — the
site's mailing-list signup already adds to the list.

courses/welcome/day-0.md

@@ -0,0 +1,16 @@
+---
+subject: "Welcome to Two Squirrels Press"
+---
+
+Hi there,
+
+Welcome to the Two Squirrels Press newsletter! We're glad you're here.
+
+From now on, this is where you'll hear about new releases, behind-the-scenes
+notes from our authors, and the occasional bit of news worth passing along.
+No spam, no noise — just the good stuff, whenever we actually have something
+to say.
+
+Thanks for reading with us.
+
+The Two Squirrels Press team