After #312, the docs folder wants to be populated.

Create an issue for each of those contribution ideas:

• move changelog, link to release tags, docker version
• add tutorial how to customize google calendar
• add tutorial how to customize microsoft outlook
• add tutorial for free/busy days
• add tutorial for: You do not need hosting: edit a web calendar directly from Thunderbird! Using My-Mailbox-Calendar, see also #206

The tutorial should have two parts:

1. How to use the source X for the OWC leading to the last section:
2. How to embed the calendar into Y

We should probably use snippets and generate the tutorial from these:

title: "How to customize Google Calendar and embed it into Wordpress"
snippets: source/google.md + embed/wordpress.md