Disable the Gensim 3=>4 warning in docs (#3346)

* disable 3=>4 docs notification

* replace migration docs by sponsorship link

docs/src/sphinx_rtd_theme/notification.html

@@ -1,6 +1,3 @@
 <div class="notification">
-  You're viewing documentation for Gensim 4.0.0. For Gensim 3.8.3, please visit the old <a id="notification" href="https://radimrehurek.com/gensim_3.8.3">Gensim 3.8.3 documentation</a> and <a href="https://github.com/RaRe-Technologies/gensim/wiki/Migrating-from-Gensim-3.x-to-4">Migration Guide</a>.
-  <script>
-    notification.href += location.pathname.replace(/^\/gensim/, "");
-  </script>
+  Gensim relies on your donations for sustenance. If you like Gensim, please <a href="https://github.com/sponsors/piskvorky">consider donating</a>.
 </div>