Add docs and examples for decoder construction from Stim DEM strings

[vedika-saravanan]

Summary

Add public documentation and an example for constructing QEC decoders from Stim detector error model (DEM) strings.

PR #571 adds support for passing raw Stim DEM text to cudaq::qec::get_decoder(...) / cudaq_qec.get_decoder(...), but the user-facing docs and examples do not yet cover this capability.

Motivation

Users should be able to discover how to initialize decoders from a Stim DEM string without reading the implementation or tests. This is especially useful for DEM-derived workflows and for DEM-native decoders such as Chromobius.

Suggested Scope

Add a public example showing:

• C++ decoder construction from a Stim DEM string using cudaq::qec::get_decoder(name, dem_text, options).
• Python decoder construction from a Stim DEM string using cudaq_qec.get_decoder(name, dem_text, **options).
• How DEM-derived defaults such as O and error_rate_vec are supplied for PCM-based decoders when not explicitly provided.
• A short note that DEM-to-detector_error_model parsing is lossy, so DEM-native decoders should consume raw DEM text through the decoder initialization path.

Possible Locations

• QEC Sphinx docs
• QEC examples directory
• Decoder API documentation

Related

Follow-up to PR #571.