Merge bitcoindevkit/bdk#554: Fix hierarchy of headers on docs landing page

Merge bitcoindevkit/bdk#554: Fix hierarchy of headers on docs landing page

364ad95e85bd09a01cb4cbf3ceb9e9fcc074c8b4 Fix hierarchy of headers on docs landing page (thunderbiscuit)

Pull request description:

  ### Description

  This PR fixes the hierarchy of headers on the lib.rs docs. I noticed it as I was reading through the docs. The new hierarchy is the following:

  - About (h1)
  - A tour of BDK (h1)
  - Examples (h1)
    - Sync the balance of a descriptor (h2)
    - Generate a few addresses (h2)
    - Create a transaction (h2)
    - Sign a transaction (h2)
  - Feature flags (h1)
  - Internal features (h1)

  ### 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:
  rajarshimaitra:
    tACK https://github.com/bitcoindevkit/bdk/pull/554/commits/364ad95e85bd09a01cb4cbf3ceb9e9fcc074c8b4

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