Simplified design, moved TOC to sidebar

* Easier navigation with TOC on sidebar
* Removed hanging borders throughout
* Improved styling to have more consistent sizes across screens
* Made body layout fluuuuuuid
* BigCheckbox now leans more towards the medium side of the spectrum
removed bullets from TOC
Tweaked appearance of color bars for H1s to remove diagonal line

Header (and all H1s, I guess) now has the same margin in Firefox as in Chrome
Added a reference page for HTML learned in the course, and a link to it in the final "Dev Tools" step.
Add basic styling