Add Merch link to header (#2161)

## Description

added a merch store link


## Meta

Merge checklist:
- [x ] Pull Request title is [short, imperative
summary](https://cbea.ms/git-commit/) of proposed changes
- [ ] The description documents the _what_ and _why_
- [ ] If this PR changes behavior or adds a feature, user documentation
is updated
- [ ] If this PR touches photon-serde, all messages have been
regenerated and hashes have not changed unexpectedly
- [ ] If this PR touches configuration, this is backwards compatible
with settings back to v2025.3.2
- [ ] If this PR touches pipeline settings or anything related to data
exchange, the frontend typing is updated
- [ ] If this PR addresses a bug, a regression test for it is added

Author: gerth2

website/index.html

@@ -61,6 +61,11 @@
               class="header-btn"
               >GitHub</a
             >
+            <a
+              href="https://www.redbubble.com/people/PhotonVision/shop?asc=u"
+              class="header-btn"
+              >Merch</a
+            >
             <a
               href="https://demo.photonvision.org"
               class="header-btn bg-primary"