Merge bitcoindevkit#840: create taproot descriptor template

e30919b Create taproot descriptor template (Vladimir Fomene)

Pull request description:

  ### Description

  This PR solves bitcoindevkit#836. This PR adds a P2TR
  descriptor template and a BIP86 taproot
  descriptor template. With this, users
  can now create a taproot descriptor with templates.

  ### 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:

  * [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:
  notmandatory:
    ACK e30919b

Tree-SHA512: ad171d29e1b22613a9fb5e6ed148d1d3002c76152df1b5aa94e24be134d08d7c90dda9560b7501a8df28e5a7076339745da627bbdde5ed6fa11eab3199c34a99