Merge bitcoindevkit/bdk#758: Add HWI example in docs

1437e1ecfe Add the hardware_signer example (Daniela Brozzoni)
1a71eb1f47 Update the hardwaresigner module documentation (Daniela Brozzoni)
0695e9fb3e Bump HWI to 0.2.3 (Daniela Brozzoni)
a4a43ea860 Re-export HWI if the hardware-signer feature is set (Daniela Brozzoni)

Pull request description:

  ### Description

  ### Notes to the reviewers

  ### Changelog notice

  - bdk re-exports the `hwi` create when the feature `hardware-signer` is on
  - Add `examples/hardware_signer.rs`

  ### Checklists

  #### All Submissions:

  * [x] I've signed all my commits
  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
  * [x] I ran `cargo fmt` and `cargo clippy` before committing

ACKs for top commit:
  afilini:
    ACK 1437e1ecfe

Tree-SHA512: 181f4d14dce11e19497fbf30e0af8de21c2c210d37129d7d879ed5670ed09a25be1c8d371389c431e18df9e76870cf5e4afe7b29a6c05fe59b3e1816bc8cf673

src/lib.rs

@@ -203,6 +203,8 @@ pub extern crate miniscript;
 extern crate serde;
 #[macro_use]
 extern crate serde_json;
+#[cfg(feature = "hardware-signer")]
+pub extern crate hwi;
 
 #[cfg(all(feature = "reqwest", feature = "ureq"))]
 compile_error!("Features reqwest and ureq are mutually exclusive and cannot be enabled together");