RFR: Add support for mermaid diagrams

[Magnus Ihse Bursie]

On Wed, 10 Feb 2021 15:40:59 GMT, Magnus Ihse Bursie <ihse at openjdk.org> wrote:

> In preparation for updates of the description on version control, I'd like to replace the hard-coded gif image diagram with a flexible system of generating diagrams using `mermaid`. Not only will it simplify replacing the mercurial diagram with a new git/GitHub diagram, but it will also make it dead simple to add more diagrams in the future.

<img width="1106" alt="Screen Shot 2021-02-10 at 16 43 21" src="https://user-images.githubusercontent.com/6209108/107533404-2f372200-6bbf-11eb-92f8-15aa2c2c0dfc.png">

This is sort of how it looks. (It is rendered locally on my machine without access to proper CSS, that's why it looks a bit weird.)

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PR: https://git.openjdk.java.net/guide/pull/45