+conda recipe to install on windows

[alex11818]

Problem

PET cannot currently be installed on Windows with pip or uv because of graphite-maps and its dependencies: PET's EnIF analysis depends on graphite-maps, which in turn needs scikit-sparse and the CHOLMOD C library. Neither package has a Windows build on PyPI; conda-forge provides scikit-sparse and CHOLMOD ready-made. import pipt loads EnIF unconditionally, so this affects every PET run on Windows.

What this PR adds

• environment.yml: a recipe to install PET and its environment with conda:

  conda env create -f environment.yml -n pet
  conda activate pet
  

• docs/installation_windows.md: prerequisites, install and check commands,
  and the pitfalls found along the way.

• A Windows note in the EnIF tutorial, a nav entry in mkdocs.yml, and a
  changelog entry.

• README: Installation is split into "On Linux" and "On Windows".

No changes to src/ or pyproject.toml.

Testing

On Windows 11 with Miniforge, in an environment built from environment.yml:

• import pipt and from sksparse.cholmod import cholesky succeed.
• All 33 tests in tests/assimilation/test_enif.py pass.
• pytest -m "not slow": 583 passed, 6 failed, 2 skipped.

The six failures are not installation problems:

• Five tests in tests/assimilation/test_subspace2.py compare two analyses bit
  for bit; on Windows they differ in the last floating-point digit (~1e-16).
• tests/test_logging_and_paths.py::test_save_folder_is_not_created_by_reading_it
  expects a / path separator and gets \.

Not tested: building the environment on Linux (its conda packages do resolve
for linux-64), and rendering the docs with mkdocs.

🤖 Generated with Claude Code, edited by @alex11818