Docs ordering b28

Created by: portnov

• Add a test case which checks that each .rst file under docs/nodes has corresponding .py file (i.e. that we do not have obsolete/hanging documentation files)
• Move documentation files around to make that test happy
• Move old nodes documentation into docs/old/nodes for now, maybe someone will discover that some of them are the best thing that we have for some undocumented node.