RFR: Add support for mermaid diagrams

[Magnus Ihse Bursie]

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.

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Commit messages:
 - Add support for mermaid diagrams

Changes: https://git.openjdk.java.net/guide/pull/45/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=45&range=00
  Stats: 36 lines in 5 files changed: 27 ins; 3 del; 6 mod
  Patch: https://git.openjdk.java.net/guide/pull/45.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/45/head:pull/45

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