Add OpenCollective transfer address to docs/donations.rst
Also update the Aspiration sections with overall details, and the
second phase we did in 2020.
This uses Read-The-Docs (sphinx/docutils) references exclusively, but adds a
README.md for GitHub viewers to remind them that the links there won't
work (closes ticket:2835).
It also fixes all the dangling references and other Sphinx warnings.
The "Preparation" section of docs/magic-folder-howto.rst was removed, since
this feature has since been merged to trunk.
