Merge bitcoindevkit/bdk#780: Update psbt_signer example to use descriptor! macro

1331193800911bb8168e5d1c2689676af3c1fe86 Update psbt_signer example to use descriptor! macro (Steve Myers)

Pull request description:

  ### Description

  This is a small fix to the psbt_signer example to also use the `descriptor!` macro.

  ### Notes to the reviewers

  I also added more docs to at the beginning of the example.

  ### Changelog notice

  None

  ### 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

  #### New Example:

  * [x] I've added docs for the new example

ACKs for top commit:
  danielabrozzoni:
    ACK 1331193800911bb8168e5d1c2689676af3c1fe86

Tree-SHA512: 602fa317313dea77bc4804abce500db33d5834625704019c6590ae6b80cf339cbaddffef667eaef2696e8e769756a2c2405c84109409ef33816db60d3df4d53d