[TTS] Add Spanish model documentation
Signed-off-by: Ryan <rlangman@nvidia.com>

# What does this PR do ?

Add new Spanish FastPitch model to available models

https://catalog.ngc.nvidia.com/orgs/nvidia/teams/nemo/models/tts_es_multispeaker_fastpitchhifigan/files

After change is reviewed, I need to update the nvidia catalog with the correct version & filenames.

**Collection**: [TTS]

# Changelog 
- Add Spanish FastPitch & HifiGan to available models and documentation.

# Usage

```python
import soundfile as sf
from nemo.collections.tts.models import FastPitchModel
from nemo.collections.tts.models import HifiGanModel

fastpitch_name = "tts_es_fastpitch_multispeaker"
hifigan_name = "tts_es_hifigan_ft_fastpitch_multispeaker"

# Load spectrogram generator
spec_generator = FastPitchModel.from_pretrained(model_name)

# Load Vocoder
model = HifiGanModel.from_pretrained(model_name=model_name)

# Generate audio
text = "Escribe tu texto aquí."
parsed = spec_generator.parse(text, normalize=False)
speaker = 5
spectrogram = spec_generator.generate_spectrogram(tokens=parsed, speaker=speaker)
audio = model.convert_spectrogram_to_audio(spec=spectrogram)
audio = audio.detach().cpu().numpy()

# Save the audio to disk in a file called speech.wav
sample_rate = 44100
sf.write("speech.wav", audio, sample_rate)
```

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [x] Documentation