megatron_bert model card update referencing #36979

[ArkVex]

What does this PR do?
This PR updates and standardizes the model card for Megatron-BERT (megatron_bert.md) to follow the latest documentation template as outlined in issue #36979.

Adds release info and feature badges
Improves the model description and usage examples
Includes quantization and attention mask visualizer sections
Adds autodoc blocks for all relevant classes
Enhances formatting and completeness for consistency with other model cards
Motivation:
This change improves the clarity, usability, and consistency of the Megatron-BERT documentation, making it easier for users to understand and apply the model.

Fixes:
Addresses the community documentation standardization request in #36979.

[ArkVex]

@stevhliu review plz

[stevhliu]

Thank you so much for the time and effort you put into this—I really appreciate it. With the number of models in Transformers growing so quickly, I’ve been looking into ways to automate model card generation to keep up with the pace. For now, I’ll be closing this PR, but I’m very grateful for your interest in this project!

Thanks again 🤗