docs: Restructure README with backend-specific quick start and setup guides

[euronymous-aithal]

What does this PR do ?

Add a one line overview of what this PR aims to accomplish.
updating readme

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Summary by CodeRabbit

• Documentation
  • README redesigned into an onboarding/architecture guide with a new Overview emphasizing scalability, multimodal support, and open-source design.
  • Added a collapsible "Previous News" block and reorganized navigation into a two-column Quick Start and Support Matrix/TOC.
  • Introduced explicit Training (DTensor, Megatron) and Generation (vLLM, Megatron) backend sections with auto-detection and config guidance.
  • Expanded prerequisites, detailed setup/run examples (single-/multi-node), GRPO and finetuning runnable examples, and clearer environment/build warnings.

[terrykong]

@euronymous-aithal did you want to fold any of this PR into yours? https://github.com/NVIDIA-NeMo/RL/pull/965/files

[euronymous-aithal]

@terrykong i folded changes from "https://github.com/NVIDIA-NeMo/RL/pull/965/files" to the current PR. please review and let me know

[terrykong]

lgtm. thanks @euronymous-aithal