docs: add shared merchandise support page (#42)

## Why

We want the site to present project support in a way that fits
cpp-linter's maintainer model. Instead of framing shirts as direct
sales, this change points people to the shared Open Collective account
so support goes to the project as a whole.

## What changed

- add a new `Merchandise` page to the site navigation
- introduce support-first copy that explains why a shared public fund is
preferred
- link directly to the live Open Collective page as the canonical
support destination
- clarify that shirts are a manual thank-you perk, not an automatic
checkout flow or purchase entitlement

## Notes for review

The page intentionally avoids fixed pricing, order intake, and
fulfillment promises. It keeps the shirt idea visible while setting
expectations around stock, shipping, and maintainer-managed
distribution.

---------

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>

Author: 3 people

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: |