Introduce .tool-versions with newest nodejs 16 and update documentation to reference this file. This way the current node version is saved in one place that can be read by runtime version managing software, eg. https://asdf-vm.com/
Also small restructuring of the installation instruction list, including https://github.com/pietervdvn/MapComplete/pull/483.
`npm run init` requires `wget` to be installed but the error message is tiny and easy to miss. Without wget preset, the init tasks does not all it needs to do.
Documenting it in the instructions explicitly works around this issue.
