Implement example: 7GUIs Cells

[tbreuss]

The implementation must be integrated in this Markdown file:

• https://github.com/tbreuss/mithril-by-examples/blob/main/examples/7gui-cells.md

And the content should be formatted like here for example:

• https://github.com/tbreuss/mithril-by-examples/blob/main/examples/routing.md

If you have any question, leave a message in the comments.