[ty] Add note to unresolved-import hinting to users to configure their Python environment (#18207)

Closes https://github.com/astral-sh/ty/issues/453.

## Summary

Add an additional info diagnostic to `unresolved-import` check to hint
to users that they should make sure their Python environment is properly
configured for ty, linking them to the corresponding doc. This
diagnostic is only shown when an import is not relative, e.g., `import
maturin` not `import .maturin`.

## Test Plan

Updated snapshots with new info message and reran tests.

crates/ty_python_semantic/src/types/infer.rs

@@ -3812,11 +3812,16 @@ impl<'db> TypeInferenceBuilder<'db> {
         let Some(builder) = self.context.report_lint(&UNRESOLVED_IMPORT, range) else {
             return;
         };
-        builder.into_diagnostic(format_args!(
+        let mut diagnostic = builder.into_diagnostic(format_args!(
             "Cannot resolve imported module `{}{}`",
             ".".repeat(level as usize),
             module.unwrap_or_default()
         ));
+        if level == 0 {
+            diagnostic.info(
+                "make sure your Python environment is properly configured: https://github.com/astral-sh/ty/blob/main/docs/README.md#python-environment"
+            );
+        }
     }
 
     fn infer_import_definition(