Add docs with non-integer index types

Either the crate-level or the Range docs should describe how to write a custom type that can be used as an index. (This will require you to impl `num_traits::One` for the custom type, for example.)

Review the crate-level documentation and the Range documentation to find the best place for an indexing example. Check how custom index types are constrained, including the need to implement num_traits::One, and document a complete example; the issue is done when users can understand how to use a non-integer index type.

Written by the indexing model from the issue text.