The responsive-font-size mixin documentation

Hello,

I just thought it was useful to report this about the documentation at https://mdbootstrap.com/docs/b4/jquery/content/responsive-font-size/

1. The PostCSS plugin link is broken
2. You should add a warning to specify that the Bootstrap `$enable-responsive-font-sizes` variable must be set to `true` in order to make it work (the default value is `false`)