add restructuredtext as markdown option

[LuisBL]

https://www.sphinx-doc.org use the reStructuredText markdown syntax https://docutils.sourceforge.io/rst.html

$ cat a.rst 
mermaid rst directive
=====================

.. mermaid::

  flowchart TB
    c1-->a2
    subgraph ide1 [one]
    a1-->a2
    end
$

would be fantastic to have it available in diagram.nvim