[docs] Fixes and style improvements for GitHub Pages doc site

[ericyhwang]

• Fix a multitude of broken links and formatting
• Switch template examples to "jinja" language that's supported by rouge-ruby syntax highlighter, with style customizations
  • The old derby-site used a custom Derby syntax highlighter written in JS. That won't work with the new Jekyll docs with rouge-ruby as syntax highlighter.
  • After some experimentation, "jinja" with some small style customizations is good enough, so I don't feel it's worth the time at the moment to write a custom Ruby lexer for Derby templates.
• Customize theme with Derby colors from derby-site
• Point main README to new GH Pages doc site
  • I'll update the old derby-site later to have its Docs header link point to GH Pages.

Example of fixed code formatting and new theme colors:

[craigbeck]

👍