Add an example using custom distance for nearest search

[aprokop]

[aprokop]

explore defining struct MyPoint { float x, y; };

The whole point is for a user to not define their own type. The only thing that's necessary is to have a separate type for disambiguation during the dispatch.

[aprokop]

@dalg24 Do you have any blocking objections to merging the example in the current form?

[aprokop]

I chose distances so that a user can see that the computed distances are indeed the custom ones. Computing just the found indices seems insufficient to me, as it still does not confirm that we can get the correct distance in the callback.

@dalg24 If you find complexity unnecessary, please provide a reply to this comment.