feat(marketplace) add a section for the marketplace from the dev documentation

Signed-off-by: Benoit Mortier <benoit.mortier@fusiondirectory.org>

source/fusiondirectory-marketplace/index.rst

+FusionDirectory Marketplace
+===========================
+
+The FusionDirectory marketplace is the best way to find new plugins for FusionDirectory. 
+
+.. image:: /_static/images/marketplace/marketplace-index.png
+
+Looking at a plugin 
+
+.. image:: /_static/images/marketplace/marketplace-show-plugin.png 
+
+
+

source/index.rst

@@ -20,6 +20,7 @@ Contents:
    fusiondirectory-tools/index.rst
    fusiondirectory-orchestrator/index.rst
    fusiondirectory-webservice/index.rst
+   fusiondirectory-marketplace/index.rst
    support/index.rst
    security/index.rst
    authors/index.rst