RFR: Add support for mermaid diagrams

[Iris Clark]

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.

Marked as reviewed by iris (Reviewer).

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