Render transitions as Mermaid

[redeboer]

GitHub now supports Mermaid diagrams in Markdown text. See Mermaid syntax specifications.

Some benefits:

• Can be rendered without Graphviz, which needs to be installed system-wide.
• Related: can be rendered in the documentation with sphinxcontrib-mermaid.
• Supports unicode.
• Can be rendered in GitHub.

An example:

```mermaid
graph LR
    A["𝐽/𝜓"] --> N0[ ]
    N0 --> N1["𝑓₀(980)"]
    N0 --> 0["𝛾"]
    N1 --> 1["𝜋⁰"]
    N1 --> 2["𝜋⁰"]
    style A fill:#FFFFFF, stroke:#FFFFFF;
    style N0 fill:#FFFFFF, stroke:#FFFFFF;
    style N1 fill:#FFFFFF, stroke:#FFFFFF;
    style 0 fill:#FFFFFF, stroke:#FFFFFF;
    style 1 fill:#FFFFFF, stroke:#FFFFFF;
    style 2 fill:#FFFFFF, stroke:#FFFFFF;
```

graph LR
    A["𝐽/𝜓"] --> N0[ ]
    N0 --> N1["𝑓₀(980)"]
    N0 --> 0["𝛾"]
    N1 --> 1["𝜋⁰"]
    N1 --> 2["𝜋⁰"]
    style A fill:#FFFFFF, stroke:#FFFFFF;
    style N0 fill:#FFFFFF, stroke:#FFFFFF;
    style N1 fill:#FFFFFF, stroke:#FFFFFF;
    style 0 fill:#FFFFFF, stroke:#FFFFFF;
    style 1 fill:#FFFFFF, stroke:#FFFFFF;
    style 2 fill:#FFFFFF, stroke:#FFFFFF;

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[shenvitor]

Interesting 👀

[redeboer]

After suggestions by @grayson-helmholz @Smaldanerep1, this may look better:

```mermaid
graph LR
    N0["𝐽/𝜓"] --> N1["𝑓₀(980)"]
    N0["𝐽/𝜓"] --> 0["𝛾"]
    N1 --> 1["𝜋⁰"]
    N1 --> 2["𝜋⁰"]
    style N0 fill:#FFFFFF, stroke:#FFFFFF;
    style N1 fill:#FFFFFF, stroke:#FFFFFF;
    style 0 fill:#FFFFFF, stroke:#FFFFFF;
    style 1 fill:#FFFFFF, stroke:#FFFFFF;
    style 2 fill:#FFFFFF, stroke:#FFFFFF;
```

graph LR
    N0["𝐽/𝜓"] --> N1["𝑓₀(980)"]
    N0["𝐽/𝜓"] --> 0["𝛾"]
    N1 --> 1["𝜋⁰"]
    N1 --> 2["𝜋⁰"]
    style N0 fill:#FFFFFF, stroke:#FFFFFF;
    style N1 fill:#FFFFFF, stroke:#FFFFFF;
    style 0 fill:#FFFFFF, stroke:#FFFFFF;
    style 1 fill:#FFFFFF, stroke:#FFFFFF;
    style 2 fill:#FFFFFF, stroke:#FFFFFF;

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