* Add error/warning when a page is rendered without a /

Throws an error for development and gives a warning in production

* Add tests for error when rendering without starting slash

* Update to always warn and add err.sh

* Update errors/render-no-starting-slash.md
Co-authored-by: NJJ Kasper <jj@jjsweb.site>
Co-authored-by: NTim Neutkens <tim@timneutkens.nl>