Add Demos and Posts sections

- Added Demos and Posts sections to page
- Reworked each section to line up the header and the content better
- Set some max sizes on elements to make site look better at very large
  and very small viewport sizes
- Added TODOs for actual viewport support
Used semantic elements where appropriate

- Replaced some divs with semantic elements
- Removed a top-level container, reduced nesting depth
Removed unused style and set size on photo

- Removed unused classes from photo
- Explicitly set size for photo to prevent a relayout