Update modernbertdecoder docs
Author: orionw
Pull request: yes

# What does this PR do?
Updates the docs for the ModernBERTDecoder class to link to the paper and to the correct model name. 


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@stevhliu for docs, if you have a sec!

Comments:
Thanks so much for the suggestions @stevhliu, added them! 

> It may also be nice to include a quantization example with the [ettin-decoder-1b](https://huggingface.co/jhu-clsp/ettin-decoder-1b) checkpoint!

I actually am not super good at quantization (others were super kind and contributed an onyx version to some models that IDK how to do), so I might just leave it for now - although I love the idea!

The snippet below should work for quantization! :)

```py
import torch
from transformers import AutoModelForCausalLM, AutoTokenizer, BitsAndBytesConfig

quantization_config = BitsAndBytesConfig(
    load_in_8bit=True,
)

tokenizer = AutoTokenizer.from_pretrained("jhu-clsp/ettin-decoder-1b")
model = AutoModelForCausalLM.from_pretrained(
    "jhu-clsp/ettin-decoder-1b",
    torch_dtype=torch.float16,
    device_map="auto",
    quantization_config=quantization_config
)

prompt = "The future of artificial intelligence is"
inputs = tokenizer(prompt, return_tensors="pt").to("cuda")

with torch.no_grad():
    outputs = model.generate(
        **inputs,
        max_length=50,
        num_return_sequences=1,
        temperature=0.7,
        do_sample=True,
        pad_token_id=tokenizer.eos_token_id
    )

generated_text = tokenizer.decode(outputs[0], skip_special_tokens=True)
print(f"Generated text: {generated_text}")
```

Awesome, thank you so much @stevhliu, added that to the [model card](https://huggingface.co/jhu-clsp/ettin-decoder-1b#usage-examples)!

Is there anything else to add for this PR or is it good to go? 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_39453). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

I think for this PR, we should still:

1. remove the ## Usage tips section at the bottom since most of the information is already incorporated in the card
2. add the quantization example to this doc as well

Perfect, sorry I missed that. Added now!