Slight changes to `MODEL_CARD.md` to organize information

[pchng]

• Move GQA section to Model Architecture as this is where it appears in the blog post section: https://ai.meta.com/blog/meta-llama-3/
• Included information about tokenizer and context length, copied from above blog post
• Consolidated information about "family of models" with "variations" and replaced the latter with the former

Totally subjective, so feel free to reject this PR.

[subramen]

The change helps improve readability, lgtm

[pchng]

Any chance of this @carljparker ? 🙏

[subramen]

Thanks for your contribution @pchng!