doc: contributing: index: give contrib by dev first (#2472)

Before this:

-> Contributing to the project
-> Maintenance
-> Example developing of a new plugin

Considering that maintenance questions are not dev questions & a meta project
questions for contributors which are promoted to maintainers.

-> Contributing to the project
-> Example developing of a new plugin
-> Maintenance

Author: Anton-Latukha

docs/contributing/index.rst

@@ -5,5 +5,5 @@ Contributing
    :maxdepth: 2
 
    contributing
-   releases
    plugin-tutorial
+   releases