docs: add rustdoc for markets read cache constructor

[silentgeckoaudit3801]

Fixes #857.

Summary

• adds the missing /// rustdoc for MarketReadCache::new
• keeps the change scoped to contracts/predictify-hybrid/src/markets.rs
• does not change contract behavior or storage logic

Validation

• Static raw-file inspection confirmed every pub fn in markets.rs now has immediate /// rustdoc.
• Compare is 1 commit ahead / 0 behind and changes only docs (+5/-0).
• Not run locally: docs-only change; this workspace is avoiding project toolchain execution.