FIX make sphinx docs work on Windows (#206)

`make` does not exist on Windows in general, causing `spin docs` to fail.
If `make.bat` exists, invoke that instead.

As noted in
scikit-learn/scikit-learn#29012 (comment)
`make.bat` is generated by `sphinx-quickstart` and some projects still
have it. scikit-learn has it and I think it is supposed to work,
although we don't have that many developers on Windows. `numpy` and
`matplotlib` still have `make.bat` as well.

spin/cmds/meson.py

@@ -857,7 +857,10 @@ def docs(
             bold=True,
             fg="bright_blue",
         )
-    _run(["make", "-C", doc_dir, sphinx_target], replace=True)
+
+    make_bat_exists = (Path(doc_dir) / "make.bat").exists()
+    make_cmd = "make.bat" if sys.platform == "win32" and make_bat_exists else "make"
+    _run([make_cmd, sphinx_target], cwd=doc_dir, replace=True)
 
 
 @click.command()