[doc] adds alabaster source

docs/_templates/alabaster/donate.html

@@ -0,0 +1,9 @@
+{% if theme_gratipay_user or theme_gittip_user %}
+<h3>Donate</h3>
+<p>
+Consider supporting the authors on <a href="https://www.gratipay.com/">Gratipay</a>:
+<script data-gratipay-username="{{ theme_gratipay_user or theme_gittip_user }}"
+        data-gratipay-widget="button"
+        src="//gttp.co/v1.js"></script>
+</p>
+{% endif %}