Clean up code & tweak presentation

[infotexture]

The recent changes from #4, #7, #10 & #11 add new features to the plug-in that improve the default output, but introduce a few inconsistencies in code formatting and presentation that should be aligned before the 5.3 release.

For example:

• Restore csspath for cleaner output folder #14
• Reformat code for consistent indentation #15
• Use valid ARIA role for ToC <nav> #16
• Update header examples #17
• Review sample header/footer content #18
  • Review links to external projects like DITA-OT
  • Align CI output with default plug-in output
• Fix invalid DITA markup #19
• Tweak default styles for neutral presentation & accessible text contrast
• Review documentation & code comments to facilitate customization

We may want to consider removing inactive code at some point, as >75% of the code on develop is currently in the unreferenced css/custom.css file with the alternate theme override.

(According to cloc, the codebase has grown from 183 lines of code in v3.6 to over 3500 on feature/attrs).