Support YAML frontmatter configuration

[kdeldycke]

Each mermaid diagram can be prefixed with a frontmatter configuration block to alter its default rendering. Example:

---
title: Hello Title
config:
  theme: base
  themeVariables:
    primaryColor: "#00ff00"
---
flowchart
	Hello --> World

This is the preferred way starting with Mermaid v10.5.0+ since %%{``}%% directives are now deprecated.

Unfortunately this syntax is not supported by sphinxcontrib-mermaid.

How to reproduce

Add the following diagram to any markdown file:

```mermaid
---
title: Hello Title
config:
  theme: base
  themeVariables:
    primaryColor: "#00ff00"
---
flowchart
	Hello --> World
```

It gets rendered with its default style:

Instead, I expect it to be rendered the same way it is displayed in the official documentation:

This issue was produced in the following environment:

• Sphinx 7.3.7
• sphinxcontrib-mermaid 0.9.2
• myst-parser 3.0.1
• Python 3.12.4

[kdeldycke]

Note that GitHub is supporting these frontmatter configuration directives:

---
title: Hello Title
config:
  theme: base
  themeVariables:
    primaryColor: "#00ff00"
---
flowchart
	Hello --> World

Loading

[kdeldycke]

My particular use-case for this was to deactivate the rendering of values in a Sankey diagram like this:

---
config:
  sankey:
    showValues: false
---
sankey-beta

Agricultural 'waste',Bio-conversion,124.729
Bio-conversion,Liquid,0.597
Bio-conversion,Losses,26.862
Bio-conversion,Solid,280.322
Bio-conversion,Gas,81.144

As a temporary workaround, I deactivated globally value rendering for all Sankey diagrams by tweaking the Mermaid initialization in my Sphinx's conf.py configuration file:

mermaid_version = "latest"
mermaid_init_js = """
  const config = {
    startOnLoad: true,
    sankey: {
      showValues: false,
    },
  };
  mermaid.initialize(config);
"""

[mgaitan]

@kdeldycke are you interested in opening a PR for this? It seems a very valuable feature.