Associations guide: Add note that you must use the build_ prefix to build associations of the belongs_to and has_one type

Added `where` as a method to has_many and has_and_belongs_to_many collections.

documents automatic management of join models in hmt associations, in particular the gotcha that deletion is direct

typo 'main.cs' -> 'main.css', and textile typo 'h6(:has_many-group)' -> 'h6(#has_many-group)', and typo. missing 'if'

Add active links to internal sections.
Use textile markup for particular words.
Improve the indentation in code examples.
Fix some typos.