#38 Improved AsciiDoctor-generated documentation HTML pages style

• Best to review per commit.
• Introduced custom AsciiDoctor stylesheet.
• Various HTML styling improvements/fixes. Also closer to the website style.
• Each docset now has its own theme, similar to the corresponding website theme.
• Reduced the size of the website by sharing CSS file between pages of each docset (mostly undone by including the full TOC, see below).
• Indicated external links with an icon after it.
• Includes rudimentary TOC interactivity (collapse/expand functionality).
• Include all TOC levels in TOCs of website pages.
• Changed Chi and ToolDef website link color to be same as documentation, and be less close to black.
• Some other changes. See separate commits.

Addresses #38 Fixes #395 (closed)