Version 4 (mdo#261)

* Update for v4

- Rewrite all content in Markdown
- Drop some outdated content
- Fix all open issues
- New project logo
- Updated design

* Add CSS Colors section for color syntax and contrast

* edit

* More code cleanup, new logical properties section, link more color stuff

* Update README.md

* Few tweaks

* Update social image, favicon, and OG and Twitter images

* Update image URL

minor: point to bootstrap repo main branch

rip gauges, https twitter widget

copyediting

Merge pull request mdo#22 from cironunes/gh-pages

fix anchor for open a new issue

v2

* Rewrote golden rule
* Added boolean attributes to HTML section
* Added media query placement to CSS section
* Added editor preferences section
* Rewrote hella other stuff
* Added some source/additional reading links

Merge pull request mdo#14 from beaucharman/ac3b00a61e9eac1d8535fa1aa5…

…4762359ad985cf

Added semicolon to the end of the css value on line 198