Revamp menus on documentation section pages

Georgia Stuartrequested to merge
better-landing-pages into main

Makes the menu bit of documentation section pages nicer.

Reads the icon and blurb params from the sub-page's frontmatter (closes #26 (closed) also):

image

Default, when no icon or blurb is specified for a page:

image

Signed-off-by: Georgia Stuart gstuart@umass.edu

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