Add structured system design prep roadmap resource (StreamPrep)

[usubhash]

Hi, thank you for this system design interview repository.

I’d like to suggest adding StreamPrep as an optional structured roadmap for learners who want pacing/progression while preparing system design and adjacent interview topics.

Why this can complement the repo:

• Repo provides strong system design materials
• StreamPrep adds weekly sequence and execution guidance

I can submit a tiny PR if this is approved.

Proposed line:
- [StreamPrep](https://streamprep.dev) - Structured interview prep roadmap with guided progression, including system design practice.