DOC: render qrules results with Mermaid

[dependabot]

• Closes Render qrules transitions with Mermaid instead of Graphviz #230
• Closes Aligned amplitude equation renders as an error under MathJax 4 #229

📝 Documentation

• Decay topologies in the notebooks are rendered with Mermaid through qrules.io.asmermaid() and IPython.display.Markdown instead of with Graphviz. Sphinx renders the fenced blocks in the notebook output through sphinxcontrib-mermaid, with myst_fence_as_directive = ["mermaid"].
• mermaid_init_config and a new docs/_static/mermaid.css keep the diagrams at their natural size instead of stretching them over the page width, and give edge labels a solid background so that edges do not shine through.
• The main amplitude equation on the front page uses an aligned environment instead of the deprecated eqnarray, which MathJax renders with correct spacing around the alignment points.
• Hidden input cells in the notebooks carry a code_prompt_show label that says what they contain, and the lineshape cells in the $J/\psi \to K^0_S p \bar{p}$, $\Lambda_c^+ \to p K^- \pi^+$ and $\Xi_b^- \to p K^- K^-$ notebooks show the full lineshape through unfold_definitions() instead of listing each sub-expression by hand.
• Progress bars and warnings from qrules.transition are muted in the notebooks, so the Sphinx output no longer contains stray logging output.

🖱️ Developer experience

• Graphviz is no longer needed as a system dependency. It has been removed from .readthedocs.yml, the CI documentation job, .binder/apt.txt, and [tool.compwa.policy], so the documentation can be built without an apt installation step.
• The notebook dependency group requires qrules>=0.10.12 for qrules.io.asmermaid() instead of the qrules[viz] extra.

Squash commit messages

* DOC: improve notebook renderings in Sphinx
* DX: drop Graphviz as a system dependency
* FIX: replace eqnarray with aligned in amplitude equation
* MAINT: increase minimal QRules version