Fix links to other versions in mkdocs documentation footer

The problem was that we didn't had access to the correct page name (the
relative path to the source file without the .md suffix).

In order for this fix to work mkdocs needs to integrate mkdocs/mkdocs#721 and
create a new release.

Author: gregmuellegger

readthedocs/doc_builder/backends/mkdocs.py

@@ -122,7 +122,10 @@ def append_conf(self, **kwargs):
 
         data_file = open(os.path.join(self.root_path, docs_dir, 'readthedocs-data.js'), 'w+')
         data_file.write(data_string)
-        data_file.write('\nREADTHEDOCS_DATA["page"] = mkdocs_page_name')
+        data_file.write('''
+READTHEDOCS_DATA["page"] = mkdocs_page_input_path.substr(
+    0, mkdocs_page_input_path.lastIndexOf(READTHEDOCS_DATA.source_suffix));
+''')
         data_file.close()
 
         include_ctx = {