Heavily revise Manifests chapter, and add example manifests

- Mention puppet agent much much earlier!
- Punt apache example
- Front-load page with action
- Improve explanations
- Add call-out to DL PE
Re-add ssh key exercise to manifests chapter
undo prompt fixes in manifests
undo prompt fixes in ral
LP: Revise "ordering" page

- Improve examples and explanations
- Add example files
- Add call-out for PE download