Merge bitcoindevkit/bdk#2007: docs(chain): add doctest for min confirmation balance...

Merge bitcoindevkit/bdk#2007: docs(chain): add doctest for min confirmation balance filtering

657a044370a102259eb1b7f5abdfee2dde998bc2 docs(chain): add doctest for min confirmation balance filtering (Wei Chen)

Pull request description:

  Implements #1942.

  ### Description

  This PR adds a new doctest demonstrating how to simulate a minimum confirmation threshold for confirmed balance calculations by adjusting the `chain_tip` height passed to `TxGraph::balance`.

  ### Changelog notice

  - Added a doctest illustrating how to filter confirmed balance results by simulating a minimum confirmation threshold via `chain_tip` height.

  ### Checklists

  #### All Submissions:

  * [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)

  #### New Features:

  * [x] I've added tests for the new feature
  * [x] 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
  * [x] I'm linking the issue being fixed by this PR

ACKs for top commit:
  oleonardolima:
    ACK 657a044370a102259eb1b7f5abdfee2dde998bc2
  evanlinjin:
    ACK 657a044370a102259eb1b7f5abdfee2dde998bc2

Tree-SHA512: 1c9514ee6cbf92df196b8e9295c81938cac4b822a7bc9703e93fabf79afaffd93d9d262bfdb74856774f1603d75b07a26cbfaefacc5a1336c8b214951458e2e1