Update document and working example on flag parsing

The current [documnetation](https://fiddle.readthedocs.io/en/latest/flags_code_lab.html?highlight=fiddler#) and working [example](https://github.com/google/fiddle/tree/main/fiddle/absl_flags/example) uses legacy flag parsing APIs.

This does not support immutable fiddlers. The example/doc could be updated to use up to date APIs for parsing/defining flags.