docs: add shared merchandise support page

docs/merch.md

@@ -0,0 +1,23 @@
+# T-shirt Thank-You
+
+We are not selling T-shirts.
+
+The T-shirt is our way of saying thank you to people who support the project. If your cumulative donations on [Open Collective](https://opencollective.com/cpp-linter) reach **USD 100**, we will reach out personally and send you one.
+
+## How to support
+
+[Support cpp-linter on Open Collective](https://opencollective.com/cpp-linter){ .md-button .md-button--primary }
+
+All contributions go to the project as a whole — not to any single maintainer — and all income and expenses are publicly visible.
+
+## How the T-shirt works
+
+1. Donate through [Open Collective](https://opencollective.com/cpp-linter).
+2. When your cumulative donations reach **USD 100**, we will contact you to arrange shipping.
+3. We confirm each shipment manually and collect your size and address at that point.
+
+Availability depends on stock, sizing, and shipping at the time — but we will always let you know before anything is sent.
+
+Have questions? Start a [GitHub Discussion](https://github.com/cpp-linter/cpp-linter/discussions) or open an [issue](https://github.com/cpp-linter/cpp-linter.github.io/issues).
+
+Thanks for supporting cpp-linter!

mkdocs.yml

@@ -9,6 +9,7 @@ nav:
   - Home: index.md
   - Getting Started: getting-started.md
   - Discussion: discussion.md
+  - Merchandise: merch.md
   # - Blog:
   #     - blog/index.md
 exclude_docs: |