History of Shenandoah garbage collector

Follow-up to #295 and sybling task to #300

I have noticed, the history of "Shenandoah" garbage collector contains a number of somewhat connected JEPs spanning from `JEP 189` where it was first introduced to most recent update `JEP 521`.

While I don't think this is the traditional "linking" demo, because it is not exactly the "incubator" => "preview" => "final" schema, the connection between them still should be mapped and somehow mentioned in comments, to give users better picture about the evolution of this garbage collector variant.

Currently, I have no clear idea about the actual task yet. I leave it open for someone daring enough to grab and shape this. If noone takes it, it will end up for me eventually.

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**NOTE:** This is an **ADVANCED** task to help with. Be sure you understand the requirements. Check previous contributions with `advanced` label and ask if you have any questions or doubts before doing work that will not be helpful.