9axis update

[DerAndere1]

Proposed documentation of configuration and G-codes for secondary axes (up to NUM_AXES 9

I am aware that using a scipt-based approach to update the configuration docs is preferred, but I wanted to write this down now. See #409 (comment) :

Quote from Thinkyhead:

I don't think it's very worthwhile to spend time on updating the existing configuration.md file because it is ridiculously out of date and I've been trying to get around to writing a script to convert the Configuration*.h files into a large number of .md files instead of putting all the configuration documentation on one page. Of course, easier said than done, but I did get a basic start on it. I'm not versed enough in Python to use that language, so I started off with Javascript plus node.js since it can also be tested in the browser.

So this PR can be closed anytime you like.