Big content update

* Add sentence case example
* Add comments example
* Tweak site styles—margin below list items, tighter line-height on
code snippets, and more padding in footer
* More consistent punctuation in list items
* Clarify use and reasoning of soft tabs
* Clarify use of optional trailing slashes and link to HTML5 spec
* Clarify use of classes first in attributes order
* Clarify use of lowercase hex values in CSS colors
* Clarify optional use of quotes in CSS attribute selectors
* Add reasons for CSS property order
* Clarify one-declaration rule sets
* Mention performance of attribute selectors in Selectors section
* Link to EditorConfig homepage
* Misc typos and grammar