The docs/ directory is special in GitHub, since it can be used to serve GitHub
Pages from, so there's a benefit to switching to it in order to expose it
directly as a website.
Updated references to it from the documentations themselves, from the
CONTRIBUTING.md file and from Meson build files.
Ultimately we should replace the relevant wiki texts with documentation
maintained as part of our project tree. This is a start. It can't
replace the wiki documentation in full yet, but I think it's already
very useful.
Filipe Brandenburger