Add Scarf pixel (#350)

### Description

As discussed with @pnbrown today, this PR adds a Scarf pixel to the
footer of the site for analytics purposes in a way that's compliant with
LF policies.

Insights will be available to any maintainers in the Valkey org on
https://scarf.sh.

### Issues Resolved

None. 

### Check List
- [x] Commits are signed per the DCO using `--signoff`

By submitting this pull request, I confirm that my contribution is made
under the terms of the BSD-3-Clause License.

Signed-off-by: aviaviavi <avipress@gmail.com>

Author: aviaviavi

templates/default.html

@@ -111,6 +111,7 @@
         Logo design by <a href="https://github.com/dizys" target="_blank">Ziyang.</a>
       </p>
     </div>
+    <img referrerpolicy="no-referrer-when-downgrade" src="https://static.scarf.sh/a.png?x-pxid=4682283c-f8b7-410c-9c31-c6d7944cee8b" />
   </div>
   <!-- Google Tag Manager (noscript) -->
   <noscript><iframe src="https://www.googletagmanager.com/ns.html?id=GTM-MFFCB7SR"