Fix README documentation and broken anchor links (#252)

**Description**
- Updated README.md to fix broken internal navigation links, corrected SDK import paths, added proper syntax highlighting to code blocks.
- Fixed training SDK imports, observability utils import path, and cluster management workflow examples.

**Testing Done**
- Verified all anchor links work correctly in table of contents and usage sections
- Cross-referenced SDK imports against actual source code in src/sagemaker/hyperpod/
- Validated CLI commands match implementation in hyp_cli.py
- Confirmed code examples use correct class names and method signatures

Author: papriwal