Renaming section twincat3 to PLC

[davorpa]

What does this PR do?

Improve repo

For resources

Description

Why is this valuable (or not)?

• There are more PLC systems than TwinCat-3 https://en.wikipedia.org/wiki/Programmable_logic_controller like SCADA, so better be generalist to improve Consistency of listings (help wanted) #4579 Improve big-file usability. Add "Return to Top" Links? #5092
• Synchronize TOC with missing listing sections.

How do we know it's really free?

For book lists, is it a book? For course lists, is it a course? etc.

Checklist:

• Read our contributing guidelines
• Search for duplicates. Completes Added course for Beckhoff's TwinCAT 3. #5517
• Include author(s) and platform where appropriate.
• Put lists in alphabetical order, correct spacing.
• Add needed indications (PDF, access notes, under construction)

Followup

• Check the output of Travis-CI for linter errors!

[eshellman]

This is a good change. Our headings are there to help users find things, and I'm pretty sure more people have heard of "PLC's" than have heard of "Twincat".