Fix incorrect section numbering (from docutils)
This extension allow to render this document
==========
Document Title
==========
.. sectnum::
Section
=====
Subsection
------------
Without number for the document title as GitHub does.
Document Title
1. Section
1.1 Subsection
Without this extension, you will get this output:
1. Document Title
1.1 Section
1.1.1 Subsection