[Help wanted] Add doc about FireRedASR v2

[csukuangfj]

#3220 added two models for FireRedASR v2, a CTC-based and an AED-based.

We need to update our doc
https://k2-fsa.github.io/sherpa/onnx/FireRedAsr/pretrained.html
to include them.

You can use #3221 and #3224 as references.

Pretrained models are at

• CTC-based: sherpa-onnx-fire-red-asr2-ctc-zh_en-int8-2026-02-25.tar.bz2
• AED-based: sherpa-onnx-fire-red-asr2-zh_en-int8-2026-02-26.tar.bz2