Updates to documentation, added better intro and duplicating page
content instructions

- Adding documentation on using the ‘Disallow page types’ feature.
- Fix links
- Re-word documentation to clarify important points.
- Add new content from Sig, tidy up existing content.
- MINOR: Formatting update & draw attention to links at the bottom.
Refactored some of the text to make more sense
Merge pull request #129 from mandrew/docupdate

Updates to documentation