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

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

1437e1ecfe663b819156d98c5e1975fb357a763f Add the hardware_signer example (Daniela Brozzoni)
1a71eb1f4736651ad82e0abd64792b6cc7b16c20 Update the hardwaresigner module documentation (Daniela Brozzoni)
0695e9fb3e41727e5732561a993411147487afd3 Bump HWI to 0.2.3 (Daniela Brozzoni)
a4a43ea86060fa0a62b47dedc7de820459b3a472 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 1437e1ecfe663b819156d98c5e1975fb357a763f

Tree-SHA512: 181f4d14dce11e19497fbf30e0af8de21c2c210d37129d7d879ed5670ed09a25be1c8d371389c431e18df9e76870cf5e4afe7b29a6c05fe59b3e1816bc8cf673