Fix the documentation build: pymdown-extensions below 12 under Python 3.10

[dmeoli]

The documentation build on Read the Docs fails since 2026-10-11 (the last build that succeeded is of 2026-10-10), on every pull request, with

TypeError: Highlight.__init__() missing 1 required positional argument: 'md'

raised by mkdocstrings while rendering the API reference. The build runs under Python 3.10 (docs/environment.yml), where the last mkdocstrings is 1.0.6, whose Highlighter does not pass the md argument that the Highlight of pymdown-extensions 12 requires; under a later Python, pip takes mkdocstrings 1.0.8, which works with it.

The docs extra now keeps pymdown-extensions below 12 under Python < 3.11 only, so that the bound does not hold back a later Python. In a clean environment, pip install -e .[docs] followed by mkdocs build gives

Python	mkdocstrings	pymdown-extensions	mkdocs build
3.10, without the bound	1.0.6	12.3	fails as above
3.10, with the bound	1.0.6	11.0.2	succeeds
3.14, with the bound	1.0.8	12.3	succeeds