doc: add reference markers for markdown and docutils install.

Used by references.txt.

Author: rouilj

doc/installation.txt

@@ -251,10 +251,14 @@ jinja2
   its TEMPLATE-INFO.txt file) you need
   to have the jinja2_ template engine installed.
 
+.. _install/docutils:
+
 docutils
   To use ReStructuredText rendering you need to have the `docutils`_
   package installed.
 
+.. _install/markdown:
+
 markdown, markdown2 or mistune
   To use markdown rendering you need to have the markdown_,
   markdown2_ (2.4.9 known to be broken, 2.3.3 known to work),