Update 'Adding node types' documentation page

[SouthEndMusic]

What
Reflect all changes to Ribasim python and the Julia core on the node adding page of the contribution docs.

How
Reflect these changes:

• The change of the schema format in the schema format in ribasim_qgis/core/nodes.py;
• Deleting the scripts in the folder python/ribasim/ribasim/node_types;

and more, see #1082 which is the most recent addition of a node type.

[SouthEndMusic]

Quite some work has already been done in this PR: #1188

[visr]

Closing this. It's not complete yet, but useful and sometimes being updated.