docs: use temp directory in doctest example

The /var/cache/myapp path requires root permissions which
causes doctest failures in CI. Changed to use tempfile.gettempdir()
which works in any environment.

Author: 27Bslash6

docs/guides/backend-guide.md

@@ -146,13 +146,14 @@ export CACHEKIT_FILE_DIR_PERMISSIONS=0o700      # Default: 0o700 (owner rwx)
 **Configuration via Python**:
 
 ```python
+import tempfile
 from pathlib import Path
 from cachekit.backends.file import FileBackend
 from cachekit.backends.file.config import FileBackendConfig
 
 # Custom configuration
 config = FileBackendConfig(
-    cache_dir=Path("/var/cache/myapp"),
+    cache_dir=Path(tempfile.gettempdir()) / "myapp_cache",
     max_size_mb=2048,
     max_value_mb=200,
     max_entry_count=50000,