Docs revisions (spelling fixes, queryset examples, expansion of Editing API and Page class reference)

*  Spellcheck of docs pages. 
*  Fixes backend "building your site" example code to include querysets per (https://github.com/torchbox/wagtail/pull/256#discussion-diff-13001063)
*  Stubbed out a `Page` class reference. I welcome any changes to these since they basically define what the Wagtail project considers "safe" properties and methods to mess with when subclassing. I just did a best guess based on the number of lines in the function.
*  Fixed a few problems with RST headings (that I may have introduced in my previous PR) in the frontend "building your site" section.
*  Stubbed out some new model recipes I saw as useful
*  Expanded the Editing API doc with more on Wagtail-provided content types and field customization.