Add Linux trademark notice to footer

Added missing trademark paragraph as pointed out by marketing to footer.
Checked rendered output in browser. No issues found.

Quick fix, no associated ticket

Signed-off-by: Katrina Prosise <katrina.prosise@foundries.io>

source/_templates/footer.html

@@ -7,4 +7,9 @@
    <button type="button" button style="margin:10px;"> Offline Version </button>
    </a>
 </div>
+
+<div class="has-text-centered is-size-7 mb-2 px-6 mx-6">
+  <p>Linux<sup>&reg;</sup> is the registered trademark of Linus Torvalds in the U.S. and other countries.</p>
+</div>
+
 {% endblock %}