doc: shorten descriptions of doc homepage grid items

The descriptions of the homepage grid items were too long and often
repetitive (e.g. there's no need to repeat "Zephyr" in almost every
descriptions). This commit shortens them to aim at having them fit on
two lines, three at most.

Also updated the look&feel slightly.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>