Documentation for the memory_leak Case Study

It would be very helpful to have some documentation for the [memory_leak](https://github.com/flutter/devtools/tree/master/case_study/memory_leaks/images_1) Case Study. There is code there but nothing to indicate how to use the tool to find the issues in the code.

The companion case study called [memory_leaks/images_1](https://github.com/flutter/devtools/tree/master/case_study/memory_leaks/images_1) has good documentation.

Start by inspecting the code in case_study/memory_leaks/images_1 and compare it with the linked companion case study's existing documentation. Document how to use the tool to find the issues demonstrated by this case study; it is done when a reader can follow the instructions to reproduce and identify those issues.

索引モデルが issue の本文から書いたものです。