docs: use custom footer with links to docs issues. Fixes #4066

docs/_templates/footer.html

@@ -0,0 +1,6 b''
+{% extends "!footer.html" %}
+
+{% block extrafooter %}
+<br/>
+Documentation defects and suggestions can be submitted <a href="https://issues.rhodecode.com/projects/documentation">here</a>
+{% endblock %}