docs: update README

SETUP.md

@@ -2,41 +2,27 @@
 
 To set up the website on your local machine, the following steps are required:
 
-1. Install __Ruby >= 2.1__ if not yet available
+1. Install __Ruby 3.x__ if not yet available
 
-    ⚠️ Note for Mac users: macOS Sierra only comes with Ruby 2.0. You must therefore install a newer one manually. The best way to do this is with tools such as [Rbenv](http://rbenv.org) or [RVM](https://rvm.io) (see [short instructions](#install-or-update-ruby) below).
-
     How to check which Ruby version is currently installed:
-   
+
         $ ruby --version
-        
-
+
+    Install Ruby 3.x according to the Jekyll website at https://jekyllrb.com/docs/installation/.
+    ⚠️ For Mac, better not use [rvm](https://rvm.io/), but go fo [chruby](https://github.com/postmodern/chruby)! This is also what Jekyll recommends at https://jekyllrb.com/docs/installation/macos/.
+
 2. Install __Bundler__ if not yet available
 
         $ gem install bundler
-        
-3. Install the necessary libraries if not yet available
-
-        $ apt install build-essential patch ruby-dev libffi-dev zlib1g-dev liblzma-dev
-
-4. Install __Bundles__ if not yet available
+
+3. Install __Bundles__ if not yet available
 
         $ bundle install
 
-5. __Start Jekyll__
+4. __Start Jekyll__
 
         $ bundle exec jekyll serve
 
     Now the website can be accessed at `http://localhost:4000`.
 
 Here you can find a documentation at GitHub about how to use Jekyll locally: https://help.github.com/articles/setting-up-your-github-pages-site-locally-with-jekyll/
-
-## Install or update ruby
-
-- **Update Ruby on a Mac**
-
-    Open the terminal and enter the following command
-
-        $ \curl -sSL https://get.rvm.io | bash -s stable --ruby
-
-    When the installation is finished, open a new terminal window and run the command `rvm list known`. This shows a list of all Ruby versions. If version 2.4 is not included, you can install it with `rvm install ruby-2.4.0`. Find out the currently used version with `ruby -v`. If it is an older version than 2.4, you can use `rvm use ruby-2.4.0` to update it.