Standardize BertGeneration model card

[nemitha2005]

What does this PR do?

#36979

Updated the BertGeneration model card to follow the new standardized format including:

• New consistent layout with badges
• Friendly description written for accessibility
• Usage examples with Pipeline, AutoModel, and transformers-cli
• Quantization example with BitsAndBytesConfig
• Updated resources section with proper links

This standardizes the BertGeneration documentation to match the new template format requested in issue #36979.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#create-a-pull-request),
  Pull Request section?
• Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
  [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
• Did you write any new necessary tests?

Who can review?

@stevhliu - Documentation lead who is managing the model card standardization project.

Anyone in the community is free to review the PR once the tests have passed.

[stevhliu]

Thanks!

[stevhliu]

Suggested change

	- BertGenerationEncoder and BertGenerationDecoder should be used in combination with EncoderDecoderModel for sequence-to-sequence tasks.
	- [`BertGenerationEncoder`] and [`BertGenerationDecoder`] should be used in combination with [`EncoderDecoderModel`] for sequence-to-sequence tasks.
	<add code example here from https://huggingface.co/docs/transformers/model_doc/bert-generation#usage-examples-and-tips>

[stevhliu]

Missing a code example that shows how to use two pretrained google-bert/bert-large-uncased checkpoints

[nemitha2005]

Thanks for the feedback! I've applied all your suggestions.