Add docs for wrapping gtk widgets for new contributors

[PhilippMDoerner]

Because I dislike having to ask the same thing twice I tend to write everything down.
So basically I had a ton of notes from wrapping the Scale widget and from our discussion surrounding that.

Given that this might be useful for any future contributors I thus then decided to clean up those notes a little and add them to the nimibook, because why not at that point.

This is the first draft on how such a chapter could look like, as per the usual all of this is freely changeable.
The point after all is to have the best introductory docs for newcomers possible to increase the likelihood of others contributing.

[PhilippMDoerner]

We can of course keep going here, but I'd want to have #94 merged first so that I can update the example application provided here first.

[can-lehmann]

From my point of view, this is actually quite close to merging already. Not sure what is more convenient for you, but at least for me it is easier to review one larger PR instead of multiple smaller ones that influence each other in various ways.

[PhilippMDoerner]

From my point of view, this is actually quite close to merging already. Not sure what is more convenient for you, but at least for me it is easier to review one larger PR instead of multiple smaller ones that influence each other in various ways.

Ohhhhh I'll note that down for the future. I tend to prefer multiple smaller PRs to keep separate things in separate boxes in my head.
If fewer larger PRs are better for you I'm happy to accommodate.

If you're happy with this as is (I resolved the last few open topics we had) I'm happy to merge this branch into #95 and close this PR.

[can-lehmann]

I merged directly into main, since the changes for #95 should be quite small & I consider this PR mostly finished at this point.