Miscellaneous improvements

[AcipenserSturio]

Style tweaks:

• Use var() for CSS colours instead of C preprocessor
• Use sans-serif for most text
• Match links to yellow accent colour
• Increase contrast in info cards
• Add page-nav class
• Minor tweaks

Repo organisation tweaks:

• Group course pages of a given language into folders, to reduce pages/ clutter and to create nicer links
• Use index.html for every page, to avoid .html in links
• Generate redirects for all old links
• Group jan Lentan's original content into a folder
• Group CI deployment into bin/

Page content tweaks:

• Use the new page-nav for the Previous page - Main page - Next page widget
• Remove the Chinese orthography link zh/0 from course languages that aren't Chinese
• Remove the **NEW** disclaimer from pages that are several years old
• Move the list of jan Lentan's original content from the main page onto a separate page
• Fix indented unordered lists that had two dots instead of one