Rename amiusing mdx page

Seemingly the dots in the filename when handled by next/link result in a
strange .txt redirect in some cases (looks similar to
vercel/next.js#48996). Not clear why, but no
good reason to use this URL and make our lives difficult.

Author: pimterry

Caddyfile

@@ -8,6 +8,7 @@
     redir /view/ / 308
     redir /docs/guides/subscription-faq/ /docs/faq/payments/ 308
     redir /docs/reference/mock-page/ /docs/reference/modify-page/ 308
+    redir /docs/reference/amiusing.httptoolkit.tech/ /docs/reference/amiusing/ 308
 
     # Redirect old download paths to new ones
     @view-thankyou-subpath path_regexp subpath ^/view/thank-you/(.+)