Add sample campaign, restyle email template to brand

campaigns/welcome/ is a sample campaign showing the campaign.md format in
practice. Restyled email-templates/campaign.html to thistleandmoth.org's
actual brand colors and fonts (EB Garamond/Source Serif 4), no images, with
web-safe fallbacks for clients that can't load webfonts.

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

campaigns/welcome/campaign.md

@@ -0,0 +1,18 @@
+---
+subject: "Welcome to Thistle & Moth"
+lists: ["Newsletter"]
+from_email: "Thistle & Moth <hello@thistleandmoth.org>"
+tags: ["welcome"]
+---
+
+{{ if .Subscriber.Attribs.first_name }}Hi {{ .Subscriber.Attribs.first_name }},{{ else }}Hi there,{{ end }}
+
+Welcome to the Thistle & Moth newsletter! We're glad you're here.
+
+From now on, this is where you'll hear about what's new, what's coming, and
+anything else worth sharing. No spam, no noise — just the good stuff, when
+we actually have something to say.
+
+Thanks for reading with us.
+
+The Thistle & Moth team