Add documentation to project

[BePo65]

Today there is only a (very) short description of esday in the readme file. As the number of plugins increases, a more detailed documentation will be needed.

There are several ways to organize this documentation:

1. with docu files in the folder of the plugin (this way, all files are in 1 place)
2. in a docs directory in the project root (the test files are organized this way)
3. in a separate project (the way that dayjs organizes the documentation)

The documentation could be e.g. in the form of html or markdown files.

I would prefer variant 2 (docs directory) with markdown files.
The markdown files can easily be read in the github web page and can be converted to html and published to github pages in a github action.

[g-mero]

I agree with you.