Style fixes in CONTRIBUTING.md

* e.g., -> e.g.
* Github -> GitHub
Style fixes in index.html

* Fix unmatched <p> tag
* Always use curly apostrophe
* "just out of the reach" -> "just out of reach"
Style fixes in about-jquery

* Curly apostrophes
* Em dashes
* Proper capitalization of abbreviations (HTML etc.)
* Linkify jsbin.com
Code style adherence in how-jquery-works.md
Fix trailing whitespace.