java/linked-list: create mentor notes

Create some mentor notes for the java/linked-list exercise.

Most importantly it should include one or more reasonable solutions (at least the general structure of a reasonable solution).

Start by locating the java/linked-list exercise and existing mentor-note conventions in this repository. Review how notes present reasonable solutions, then add mentor notes for this exercise with at least the general structure of one or more reasonable solutions; done means the notes are complete and consistent with neighboring exercises.

Written by the indexing model from the issue text.