Add links to plugin guide because its buried under Docs -> Internal. At
least one user couldn't find it:
https://groups.google.com/d/msg/nomad-tool/5sR8MTGZFrM/8bOExpcJAgAJ
Link from:
- Devices index
- Devices community page
- Drivers index
- Drivers community page
I erred on the side of over-linking because it's just a single concise
sentence and I'd hate for people to struggle finding the guide.
Also remove some very old (4 years!) and confusing docs from the driver
index page.
