deps: pin pymdown-extensions to >=11.0.1 in docs group

[katsugtgz]

Pins pymdown-extensions in the docs group to address GHSA-9xwg-3r6f-jcx2, GHSA-gm37-52c6-37mw (same records as PYSEC-2026-3609, PYSEC-2026-3654).

Evidence:

• pyproject.toml docs group previously had "pymdown-extensions" with no version constraint, so a fresh uv sync could resolve any version, including affected ones
• osv-scanner reports the IDs above for pymdown-extensions below 11.0.1 (GHSA-gm37-52c6-37mw is high severity ReDoS, fixed in 11.0.1; GHSA-9xwg-3r6f-jcx2 is path traversal in the b64 extension, fixed in 11.0.0)
• new constraint: >=11.0.1; python_version >= '3.10' (11.0.1 requires Python 3.10+, matching the docs toolchain)

Validation:

• uv sync --group dev and uv sync --group docs resolve cleanly with the pin; resolved version is 11.0.1
• uv run --group docs zensical build succeeds with the pin, same 6 warnings as on main
• uv run --group dev pytest shows the same results as on main (31 failed, 256 passed, 48 skipped); the failures are pre-existing ImportError failures from missing optional torch and ultralytics extras, identical on unmodified main

Scope: dependency pin in pyproject.toml only.