examples: restructure examples folder

[AnnsAnns]

Contribution description

After #20957 we added a proper structure to the examples folder. This wasn't yet reflected in the actual folder structure within the examples folder. After positive feedback in the Matrix channel I have opened this PR in hopes of adjusting that.

This makes it far easier for newcomers to find what they are looking for and also draws more attention onto the examples folders README which explains the examples in greater detail.

To make it a bit easier to properly view the adjusted structure I'd personally recommend directly looking at the checked out branch at https://github.com/AnnsAnns/RIOT/tree/restructureExamplesFolder/examples

Testing procedure

There are no required verification steps as this doesn't modify any code aside from the RIOT_BASE variable within each examples Makefile, I did however have to adjust two CI tests to properly deal with the new examples structure. One of them being a specific examples README check, the other one relying on a fixed hello-world examples path.

[OlegHahm]

I'm gonna rephrase some of my own comments from Matrix over here:
1.) Thanks a bunch for the great proposal and all the effort. It's certainly a major improvement.
2.) I think there are various dimensions on how we could substructure the examples. This PR takes pretty good care of reflecting one of them. Maybe we could introduce alternative structures using symlinks later on.
3.) For now I think this proposal could me be merged more or less as is (need to look into the details) and improvements/adjustments can be made later on.

[MrKevinWeiss]

Could we wait until after the hard freeze, I would expect that the paths would need to be updated in the release specs as well (haven't looked into it though).