Copyedit on Developer styleguide and How to

Noticed the `#pageContainer` was unusually centred on the [CSS Code Style Guide](http://www.thinkupapp.com/docs/contribute/developers/writecode/styleguide/css.html) but could not work out to left align it. 

So just did normal sentences.

- Fix intro on PHP to match up with CSS and JavaScript.
- Reword leftover STRIKEOUT to better roll the sentence in itself
- Only need to link to MVC once per sentence.
- Formatting for a few inline code instances to help distinguish
- Brings some more consistency in how codeblocks are formatted