[Documentation] Re-structure and update README

[thvasilo]

Issue #, if available:

Fixes https://github.com/awslabs/graphstorm/issues/1262

Description of changes:

• Add status badges
• Align quick start examples with main documentation, simplify/condense instructions
• Add blog post links
• Remove outdated information/commands
• Recommend using SageMaker AI for easier distributed training
• Add build/ and dist/ directories to .gitignore

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