Sunday, 2021-07-05: This is a repost from the FreeCol forum to preserve this guide. It includes a few additional improvements.
Many thanks to Blake for initially having me write this, trying everything out causing the improvements and encouraging me now to post the news!

Stay tuned for the follow-up: The Hidden Story Of The Website Update!

How To Use Jekyll For The FreeCol Website

Please, follow these steps! If you don’t try it can never work.

I just tried every step on my second computer to make sure it works. I’d like to see if you can follow this before I edit it into the manual:

Installation

This is done only once:

• If you are on Windows go to https://rubyinstaller.org then download the recommended version and install it (the Ruby people really made an effort to make that easy, so please try and just use the standard options and wait for it to complete – do not try to install Ruby in a path with spaces; in the command window which opens for the Devkit install you should only type Enter once at start and once it completed)
• If you are on Linux use your package manager to install Ruby
• Open a command line window, type ruby -v, type Enter and check if it successfully outputs the ruby version
• Type in gem install jekyll bundler wdm and Enter to install Jekyll; it should complete without an error (if not please ask)
• You could try typing in jekyll -v to see if it got installed correctly

Use

This is done every time you want to see the effect of your edits:

• Open a command line window for the www.freecol.org directory where you put your local clone of the freecol git repository (if you have git installed on Windows just find the directory in Windows Explorer and right click and choose Git bash here; otherwise you would need to navigate through cd .. and cd _directoryname_ and show contents using dir (Windows) or ls (Linux) to get to the right directory)
• If you are inside the www.freecol.org directory you just type jekyll serve to compile the website and start a local server on this computer
• Open your browser and type 127.0.0.1:4000 in the address bar (thats the address of this local computer of yours with the port of the local server started by above command)
• Edit the website files as you like (Jekyll automatically recompiles it as long as you did not stop it or close the command window), reload (F5) in browser to see your changes
• You can stop the local server using Ctrl+C to not have it use system recources of your computer when you take a break or need a long time editing and restart it using same jekyll serve command
• jekyll build only compiles the website without starting the local server, in case you want to inspect the compilation result inside the _site directory or upload it to the webserver
• In case you accidently started jekyll serve or jekyll build in the wrong directory use jekyll clean to let it clean up the temporary files it created

Please, tell if everything worked!