LP: Revise "ordering" page

- Improve examples and explanations
- Add example files
- Add call-out for PE download
Add apache exercise answers
LP: Update variables chapter

- Minor edits
- Add many links to Puppet reference manual
- Add call-out to PE download
- Add answer to NTP exercise and build env exercise
Remove "latest" section from "index"
Tweak to variables: add a 'begin' block
Make variables-ntp.pp stop looking into the modules folder for its source files
LP: Update modules1 page

- Rewrite text for great justice
- Add examples of NTP class
- PE download callout

Still missing:
- Example NTP module
- Answer apache module