Fixing build errors caused by network support

[jpalm3r]

This pull request introduces compatibility checks and workarounds related to Python 3.14, primarily due to the pythonnet and mikeio1d dependencies not yet supporting this version. The main changes ensure that the codebase handles environments running Python 3.14, both in CI and in user-facing code, and that tests are skipped or adjusted accordingly.

Python 3.14 compatibility adjustments:

• Updated the GitHub Actions workflow (.github/workflows/docs.yml) to use Python 3.13 for documentation builds, with a comment explaining the dependency limitation.
• In Network.from_res1d (src/modelskill/network.py), added a runtime check that raises NotImplementedError if Python 3.14 or newer is detected, with a clear error message about the version requirement.
• Added sys import in src/modelskill/network.py to support version checking.

Testing adjustments:

• In tests/test_network.py, added a test (test_open_res1d) that is skipped on Python 3.14 or newer, ensuring tests do not fail due to unsupported dependencies. [1] [2]
• Updated the notebook test skip list (tests/notebooks/test_notebooks.py) to include Collection_systems_network, with a comment explaining the Python 3.14 limitation.