feat: replace hand-written CSS with Sass compilation and update site design with new fonts and layout

- Delete doc/site/style.css and add doc/site/style.scss with Sass variables, Dosis header font, box-sizing, absolute header positioning, and link hover effects
- Update template.html to reference Dosis font, lowercase nav headings, and change tagline to "a classy little scripting language"
- Add .sass-cache/ to .gitignore and rename Makefile phony target from docs to builtin
- Modify generate_docs.py to compile style.scss via sass subprocess instead of copying static CSS
- Tweak performance.markdown and qa.markdown for minor wording improvements

doc/site/qa.markdown

@@ -46,7 +46,7 @@ Here's the same example in Wren:
 The [performance page](performance.html) has more details, but the short answer is that bytecode is a nice trade-off between performance and simplicity. Also:
 
  *  Many devices like iPhones and game consoles don't allow executing code generated at runtime, which rules out a JIT.
- *  I think fibers are a really powerful tool, and implementing them is straightforward in a bytecode VM which doesn't use the native stack.
+ *  I think fibers are a really powerful tool, and implementing them is straightforward in a bytecode VM that doesn't use the native stack.
 
 ## What about your other languages?