Merge bitcoindevkit/bdk#782: Make psbt mod public and add required docs

34987d58ec9933af455e44d20c8d889bc896f869 Make psbt mod public and add required docs (Steve Myers)

Pull request description:

  ### Description

  Make psbt mod public and add required docs. The module needs to be public so `bdk-ffi` can expose the new PSBT `fee_amount()` and `fee_rate()` functions.

  ### Notes to the reviewers

  I should have done this as part of #728.

  ### Changelog notice

  Make psbt module public to expose PsbtUtils trait to downstream projects.

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

ACKs for top commit:
  rajarshimaitra:
    Concept + tACK 34987d58ec9933af455e44d20c8d889bc896f869

Tree-SHA512: 99e91e948bccb7593a3da3ac5468232103d4ba90ad4e5888ef6aebb0d16511ad3a3286951779789c05587b4bb996bc359baa28b0f4c3c55e29b24bfc12a10073