Improve dev documentation

Dev documentation has a few flaws that users could run into when using ocis for the first time.

Summary:
- Installing pnpm alone is not enough, Node.js needs to be installed as well.
- Installing any Node.js is also not enough. Ubuntu 22.04 provides version v12.22.9, which is too old. There are several ways how to update it, but maybe the best one is here: https://deb.nodesource.com/
- `OCIS_INSECURE` always needs to be set, even if `ocis init` was already executed with `insecure` flag
- `OCIS_URL` needs to be set always

For background information see this ticket: https://github.com/owncloud/ocis/issues/7391