docs(wallet): add example usage of descriptor and plan

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Description

Example usage of a vault policy, descriptor, and spend plan with a BDK wallet.

Changelog notice

docs(wallet): add example usage of descriptor and plan

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
• [x] I've added docs for the new feature (they are docs)

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