Update linux docs to be simpler

[richlander]

There are two big things that are missing from the docs today:

• Here are the packages you need
• You really need to register the PMC feed

These points are getting lost (including in the troubleshooting section) with the immediate version-specific sections.

I'm writing this note to remember this feedback. I'm happy to make these changes.

[adegeo]

What is the PMC feed?

[adegeo]

Closing this as fixed as @richlander and I worked on rewriting the linux docs