[#839] Minor edits to stats extension docs

- Edit the opening sentence, it's the stats extension that analyzes the db
  not CKAN (this is picky)
- Also mention that there are tables and graphs!
- Remove weird capitalization in bullet list
- Put built-in page view tracking before ckanext-googleanalytics and tweak
  the link to read a little better
- Add an example of ckan.plugins = stats, also cross-ref to the ckan.plugins
  docs
- Add link to stats page on demo