Break circular dependency between soroban-sdk and soroban-token-sdk. (#1558)

### What

Break circular dependency between soroban-sdk and soroban-token-sdk.

We only introduced it in order to be able to run the doctest in
migration guide soroban-sdk. I've moved the migration guide to the
respective crate (soroban-token-sdk). This may slightly reduce
discoverability, but I've left a reference to it in the main SDK
migration guide.

### Why

It's apparently impossible to publish crates with circular dependency
between them, even the dependency only occurs in dev-dependencies (or at
least I wasn't able to quickly find a way around that).

### Known limitations

N/A

Author: dmkozh

Cargo.lock

@@ -46,7 +46,6 @@ soroban-env-host = { workspace = true, features = ["testutils"] }
 soroban-ledger-snapshot = { workspace = true, features = ["testutils"] }
 stellar-xdr = { workspace = true, features = ["curr", "std"] }
 soroban-spec = { workspace = true }
-soroban-token-sdk = { workspace = true }
 ed25519-dalek = "2.0.0"
 rand = "0.8.5"
 ctor = "0.2.9"

soroban-sdk/Cargo.toml

@@ -2,7 +2,9 @@
 //!
 //! 1. [`contractevent` replaces `Events::publish`][v23_contractevent]
 //!   
-//! 2. [`MuxedAddress` replaces `Address` as the `to` of the `TokenInterface::transfer`][v23_token_transfer]
+//! 2. [`MuxedAddress` replaces `Address` as the `to` of the `TokenInterface::transfer`].
+//!    This change concerns `soroban-token-sdk` and is documented in detail in
+//!    `soroban-token-sdk` crate migration guide.
 //!
 //! # Migrating from v21 to v22
 //!
@@ -242,4 +244,3 @@
 //! [`Hash<32>::to_bytes`]: crate::crypto::Hash::to_bytes
 
 pub mod v23_contractevent;
-pub mod v23_token_transfer;

soroban-sdk/src/_migrating.rs

@@ -13,6 +13,9 @@ rust-version.workspace = true
 [lib]
 doctest = false
 
+[features]
+docs = []
+
 [dependencies]
 soroban-sdk = { workspace = true }
 

soroban-token-sdk/Cargo.toml

@@ -0,0 +1,5 @@
+//! # Migrating from v22 to v23
+//!
+//! 1. [`MuxedAddress` replaces `Address` as the `to` of the `TokenInterface::transfer`][v23_token_transfer]
+
+pub mod v23_token_transfer;

soroban-token-sdk/src/_migrating.rs

@@ -1,4 +1,6 @@
 #![no_std]
+#![cfg_attr(feature = "docs", feature(doc_cfg))]
+pub mod _migrating;
 
 use crate::event::Events;
 use crate::metadata::Metadata;