Start fixing Jekyll's broken markup and CSS.

Jekyll's default autogen starter site is garbage. Non-semantic hybrid
HTML4/5 markup, crufty CSS boilerplate, poor judgment in use of on-page
language features (like choosing SVG for a simple icon, which will break
in something like 25% of browsers) and the list goes on. Nice work,
Jekyll devs.

1. Don't use SVG features in a default starter theme for a mass
audience.
2. Don't use obscure vendor-specific classes. Let users add those if
they want them.
3. Use <main> to indicate start of content.
4. Fix nonsensical <article> layout/nesting.
5. Start ripping out CSS bloat. Probably quicker just to trash it all.