This should include the important info from the wiki.
We should try to find some recommendation for tools, or even an online
platform where people can submit translations without having to go
through all this setup
Mostly this tries to give logical header levels, so the "Fish Style
Guide" section is in the "Code Style" section
Also remove a few unimportant C++-centric sections - I'm not sure iwyu
even runs anymore, and cppcheck isn't great in my experience.
This adds a section on completions *first* and removes all mentions of
oclint as it appears to be dead.
The Vim configuration section seems to be likely to be outdated and we
don't *really* use doxygen anymore.
We use sphinx with rst for our documentation, and github supports rst
here, so it seems weird to have markdown just for these.
It also allows us e.g. to include the CHANGELOG in the docs without
requiring another build dependency.
