docs(tx-macros): document TransactionEnvelope derive, attributes, and features

[phrwlk]

This change fills the previously empty crates/tx-macros/README.md, which is included as crate-level documentation via #![doc = include_str!("../README.md")] in crates/tx-macros/src/lib.rs. The README now precisely documents the TransactionEnvelope derive macro based strictly on the current implementation in src/lib.rs, src/parse.rs, and src/expand.rs. It explains when to use the macro, enum-only and single-field variant requirements, container attributes (tx_type_name, alloy_consensus, typed, serde_cfg, arbitrary_cfg), variant attributes (ty, flatten, and optional per-variant typed), and the generated items including the ...Type enum and implementations of Transaction, Typed2718, Encodable2718, and Decodable2718. It also details serde tagging behavior for EIP‑2718 type IDs (lowercase hex with accepted aliases), provides minimal usage examples, notes path override behavior for alloy_consensus, and includes troubleshooting for common compile-time errors. This is a documentation-only change that aligns with the existing code paths and feature flags

[mattsse]

we can use crate docs here