Update design overview to reflect submitted proposals (carbon-languag…

…e#3180)

Incorporates proposals: carbon-language#1885, carbon-language#2138, carbon-language#2188, carbon-language#2200, carbon-language#2360, carbon-language#2760, carbon-language#2964,
and carbon-language#3162.

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Co-authored-by: Richard Smith <richard@metafoo.co.uk>