Add mnemonic_to_descriptors example

[vladimirfomene]

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
• [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