Modify the index page to no longer display a table of contents

The index page now acts as a sort of cover.

Add centering for images in theme.css
Add Wasp logo/mascot to index page
Modify [overview.terminology] to use a glossary.

TODO: [overview.terminology] needs some terms removed (they come into
existence in [script-structure] or [types-and-values]), and other terms
added. Specifically 'invoke', 'command', 'implementation', and possibly
a few others.