docs: refactor messy README into structured docs/

- The PLDM README was cluttered and hard to follow
- Split content into separate, focused documents under docs/
- Improves readability, structure, a

docs: refactor messy README into structured docs/

- The PLDM README was cluttered and hard to follow
- Split content into separate, focused documents under docs/
- Improves readability, structure, and long-term maintainability

Change-Id: I0236e7bdc870bb218ee09edccbeefc13bd4a637b
Signed-off-by: Manojkiran Eda <manojkiran.eda@gmail.com>

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