Feat/docs updates

[Greymantron]

Summary

This PR addresses several documentation gaps and recurring questions in the README.md to improve the onboarding experience for new contributors and users.

Changes included:

• Progress Basis Points: Clarified that the progress metric is reported in basis points (0-10000) and provided an explicit percentage conversion example. Noted that zero-total streams also report full progress.
• Troubleshooting Section: Added a new section documenting common compilation and deployment errors (wrong target, missing toolchain component, unfunded identity) along with their respective fixes.
• FAQ Section: Addressed 5 frequently asked questions regarding the contract's architecture, such as why funds are locked up front, what happens if off-chain servers disappear, and the lack of an admin freeze capability. These answers are linked directly to THREAT_MODEL.md.
• MSRV Clarification: Explicitly stated the Minimum Supported Rust Version (MSRV) as 1.84.0 in the Building section, noting that soroban-sdk requires a recent toolchain for the wasm32v1-none target.

Closes #133
Closes #135
Closes #136
Closes #137

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