docs: generate index and add it to globaltoc sidebar (#6808)

Author: blueyed

doc/en/_templates/globaltoc.html

@@ -21,3 +21,7 @@ <h3><a href="{{ pathto(master_doc) }}">{{ _('Table Of Contents') }}</a></h3>
   <hr>
   {{ toc }}
 {%- endif %}
+
+<hr>
+<a href="{{ pathto('genindex') }}">Index</a>
+<hr>

doc/en/conf.py

@@ -208,7 +208,7 @@
 html_domain_indices = True
 
 # If false, no index is generated.
-html_use_index = False
+html_use_index = True
 
 # If true, the index is split into individual pages for each letter.
 # html_split_index = False