Add a link to all install documentation files describing how to obtain a registration token

## What does this MR do?

Updates all installation documentation files with a link to Runners documentation where it is described how to obtain the registration token.

## Why was this MR needed?

## Are there points in the code the reviewer needs to double check?

## Does this MR meet the acceptance criteria?

- [x] Documentation created/updated
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)

## What are the relevant issue numbers?

Replaces !126

See merge request !362