Klondike Tutorial, Step 5: Add code and further explanation of FlatButton and others, or at least a link to its code #19

### What could be improved

As I was following Klondike's instructions, near the end of step 5, I felt like the instructions writer was not as patient as in the previous steps. Many parts I found were not explained thoroughly, there is no code for FlatButton (I know there is code for it at the end, but there is no explanation about that).
I think we need to add some more detailed explanations, if there is no code for it then at least link to the code of those parts

### Why should this be improved

We can learn flame more easily

### Risks

No risk

### More information

Step 5 at: https://docs.flame-engine.org/latest/tutorials/klondike/step5.html

### Other

- [X] Are you interested in working on a PR for this?