Mermaid experiment - draft not for merging

[ijmitch]

This PR is purely opened as a means to discuss the experimental use of Mermaid GitGraphs in place of the Powerpoint originated PNGs.

[ijmitch]

Compared to the PNG we only get the core Git information and lose the ability to include where builds happen and how testing happens.

But once you establish the principle that every branch can at least run a pipeline build, branches must build error-free as a condition of merging and every build can be deployed to a test environment, what else is left to say?

[ijmitch]

The last two commits workaround the unexpected routing and colour choice when two merges are immediately adjacent:

Adding a commit in between gives a less surprising outcome:

[ijmitch]

I've asked in the Mermaid Slack if anyone can explain our surprise at the line route and colour which I just worked around - https://mermaid-talk.slack.com/archives/CL256P19N/p1696494561191749

[ijmitch]

Discussed in the Mermaid Slack and agreed to open a new issue mermaid-js/mermaid#4912