Update documentation

* Add `warn(missing_docs)` for `bdk_wallet` and `bdk_chain`.
* Add missing documentation.
* Remove `LocalChain::heights` method.
* Remove old TODOs.

crates/chain/src/indexed_tx_graph.rs

@@ -1,3 +1,7 @@
+//! Contains the [`IndexedTxGraph`] structure and associated types.
+//!
+//! This is essentially a [`TxGraph`] combined with an indexer.
+
 use alloc::vec::Vec;
 use bitcoin::{OutPoint, Transaction, TxOut};