Merge bitcoindevkit/bdk#1896: ci: remove not needed nightly_docs workflow

2cba47399da81c0101c307a27b290d83d420d180 ci: remove not needed nightly_docs workflow (Steve Myers)

Pull request description:

  ### Description

  I removed the nightly_docs workflow since it's not needed. The latest published bdk crate docs can be found on docs.rs.

  ### Notes to the reviewers

  We early in the project life we published nightly docs since the API was changing often and releases were infrequent. But now that the api is more stable and we make regular releases nightly docs aren't needed.

  ### 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

ACKs for top commit:
  thunderbiscuit:
    ACK 2cba47399da81c0101c307a27b290d83d420d180.

Tree-SHA512: 69656d26bde1d357f823d0ba59deb4d27cee03888f578db4727f10c41086d51a4aaf592178c2467fddb95d8b80897dacc2e6824cd8a2e1d10b36ed7bbf567679