[release] Release notes for 1.76.0

From: Marshall Clow (mclow.lists_at_[hidden])
Date: 2021-03-11 16:02:05

If you have information that should go into the release notes, now’s a good
time to add it.

Create a PR against
https://github.com/boostorg/website/blob/master/feed/history/boost_1_76_0.qbk

— Marshall