Add `docs/checks.md` section for `forbidden-std-imports` check

[lonerthefirst3-sudo]

Summary

The forbidden-std-imports check is implemented in crates/checks/src/std_imports.rs but has no section in docs/checks.md.

What to add

• What it detects — use std::… import paths inside files that also contain #[contractimpl] or #[contract]
• Why it matters — Soroban contracts compile to WASM with #![no_std]. Importing from std causes a compile error in WASM targets and indicates the contract cannot be deployed as-is
• Severity — High
• Limitations — file-level check only; does not detect transitive std usage through re-exported types
• Fixture — (to be added, see issue Add test fixtures for forbidden-std-imports check (safe + vulnerable crates) #117)