Incorrect documentation links and code for ImmutableKdTree

[ozwaldorf]

There are some incomplete and/or copy pasted documentation for the new ImmutableKdTree.

• In the crate's top level doc comment, the link for the immutable tree is invalid
• https://docs.rs/kiddo/3.0.0/kiddo/immutable/float/kdtree/struct.ImmutableKdTree.html#method.size has some example code that uses the normal KdTree

[sdd]

Thanks for spotting this, and thanks even more for the PR. Will merge ASAP