Add steps for document of getting dataset 'SF Bilingual Speech'
# What does this PR do ?

When following the [document](https://docs.nvidia.com/deeplearning/nemo/user-guide/docs/en/main/tts/datasets.html#sfspeech-chinese-english-bilingual-speech) to get dataset `SFSpeech Chinese/English Bilingual Speech`, the first command
```bash
python scripts/dataset_processing/tts/sfbilingual/get_data.py \
    --data-root <your_local_dataset_root> \
    --val-size 0.1 \
    --test-size 0.2 \
    --seed-for-ds-split 100
```
 directly raise error:
```
Traceback (most recent call last):
  File "/home/xxx/NeMo/scripts/dataset_processing/tts/sfbilingual/get_data.py", line 122, in <module>
    main()
  File "/home/xxx/NeMo/scripts/dataset_processing/tts/sfbilingual/get_data.py", line 116, in main
    __process_data(
  File "/home/xxx/NeMo/scripts/dataset_processing/tts/sfbilingual/get_data.py", line 91, in __process_data
    entries = __process_transcript(dataset_path)
  File "/home/xxx/NeMo/scripts/dataset_processing/tts/sfbilingual/get_data.py", line 65, in __process_transcript
    with open(file_path / "text_SF.txt", encoding="utf-8") as fin:
FileNotFoundError: [Errno 2] No such file or directory: 'tryit/text_SF.txt'
```

The reason: `scripts/dataset_processing/tts/sfbilingual/get_data.py` actually doesn't download the dataset. The `SFSpeech Chinese/English Bilingual Speech` could only be downloaded through [Nvidia NGC](https://catalog.ngc.nvidia.com/orgs/nvidia/resources/sf_bilingual_speech_zh_en).

# Changelog
- Add steps in the document of dataset SFSpeech to download by ngc-cli tool at first

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**PR Type**:
- [ ] New Feature
- [ ] Bugfix
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## Who can review?
@blisc  @okuchaiev @titu1994 @XuesongYang 