DOC: render qrules transitions with Mermaid

[dependabot]

• Closes Equations from multiline_latex render as an error under MathJax 4 #585
• Closes Render qrules transitions with Mermaid instead of Graphviz #587
• Closes Group the test suite with pytest-describe #588

📝 Documentation

• The qrules transition graphs in docs/amplitude-analysis.ipynb and docs/amplitude-analysis/analytic-continuation.ipynb are now rendered with qrules.io.asmermaid() and IPython.display.Markdown instead of qrules.io.asdot() and graphviz.Source. Sphinx picks the diagrams up through sphinxcontrib-mermaid with myst_fence_as_directive = ["mermaid"] and nb_render_markdown_format = "myst". A mermaid_init_config and a new docs/_static/mermaid.css keep the diagrams at their natural size and give edge labels a solid background. Graphviz remains a system dependency, because sympy.dotprint() expression trees have no Mermaid equivalent.
• Display math built with sp.multiline_latex() rendered as Erroneous nesting of equation structures under MathJax 4, which Sphinx 8.2 loads by default: align* and eqnarray may not be nested inside the equation/split wrapper that Sphinx and IPython.display.Math produce. docs/usage.ipynb, docs/usage/binned-fit.ipynb and docs/usage/caching.ipynb now use ampform.io.aslatex(), which renders into the legal aligned environment.
• The documentation footer no longer names an author.

🖱️ Developer experience

• The test suite is grouped with pytest-describe: Test* classes and flat test_* functions are replaced by describe_<unit>() blocks whose cases are it_<behavior>() functions, so that a node ID such as tests/data/test_transform.py::describe_SympyDataTransformer::it_caches_the_lambdified_function names both the unit under test and the behavior that broke. Nested describe_* blocks host fixtures that are shared by one group of cases only, which removes set-up that was copy-pasted between test functions.
• Estimator stubs, the callback mock, and the polynomial minimization cases that test_gradient.py, test_minuit.py and test_scipy.py share are collected in a new tests/optimizer/__init__.py.

🔨 Maintenance

• docs/conf.py no longer filters the redundant RemovedInSphinx10Warning.

Squash commit messages

* DOC: remove author from documentation footer
* DOC: render multiline equations with ampform.io.aslatex
* DX: define tests with pytest-describe
* MAINT: remove redundant `RemovedInSphinx10Warning`