#676 PLCgen: Add documentation.

Some plcgen documentation.

Follows the previous art somewhat, but the ideas are different, so rewrote it almost completely.

Not sure the S7 and twincat pages are very useful, at least twincat is ancient (2010), I doubt you'd even get that running today.

Probably lots is wrong, but I can't focus anymore. I spend my time fighting 2FA, and I am not winning. It's filling my entire head running in circles and getting nowhere.

Addresses #676 (closed)