Improve max widths docs

[lachlanjc]

• Update class name structure
  • Min/max/fit/fill classes, .mw10, and 25/50/75% classes don't exist
• Add examples for .mw7, .mw8, and .mw9 (oddly the demo table only went 1–6)
• Better whitespace in the table
  • Space between the class name and example block
  • Padding in the example block around text
  • Padding in the .mw-100 block around text

Ideally these would be separate PRs, and I could separate them if there are any issues, but... 😉