let's add an env var for this, as this really shouldn't be a top-level
feature, as it turning off the validity checks certainly isn't
advisable.
Fixes: #4925
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.
Lennart Poettering