Merge bitcoindevkit/bdk#746: Add mnemonic_to_descriptors example

df905a8d5ef8e275188389486ac685b29f0d8c9f Add mnemonic to descriptors example. (Vladimir Fomene)

Pull request description:

  ### Description

  Using *bdk-cli* it is simple for a new user to generate a mnemonic phrase and descriptors. This might
  not be clear for new users when using *bdk* itself.

  ### Notes to the reviewers

  This was initially requested by one user but might be relevant for other users as well.

  ### 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 Features:

  * [ ] I've added tests for the new feature
  * [ ] I've added docs for the new feature

  #### Bugfixes:

  * [ ] This pull request breaks the existing API
  * [ ] I've added tests to reproduce the issue which are now passing
  * [ ] I'm linking the issue being fixed by this PR

ACKs for top commit:
  notmandatory:
    ACK df905a8d5ef8e275188389486ac685b29f0d8c9f

Tree-SHA512: ccaab775d664d1d5ad6f4cd4fb11f9552c4662fbac747ad0487a016fd4a0fa6f13fcb4dffdc6f0d2f14ee432fcb2a72a8b8837b0a977ded649ec7d3d84e1c0ed