Edit external links to open in new tabs

These external links were opened in same tabs. I think they should be opened in new tabs just like other external links.

docs/_includes/footer/about.html

@@ -7,7 +7,7 @@ <h4 class="bd-footer-title">
 </div>
 
 <div class="bd-footer-tsp">
-  Source code licensed <a href="https://opensource.org/licenses/mit-license.php">MIT</a>.
+  Source code licensed <a href="https://opensource.org/licenses/mit-license.php" target="_blank">MIT</a>.
   <br>
-  Website content licensed <a rel="license" href="https://creativecommons.org/licenses/by-nc-sa/4.0/">CC BY-NC-SA 4.0</a>
+  Website content licensed <a rel="license" href="https://creativecommons.org/licenses/by-nc-sa/4.0/" target="_blank">CC BY-NC-SA 4.0</a>
 </div>