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

Tree-SHA512: b4439a200c320a1815c572ac36b81c4d684209cb5cf1b90d3f761893ad672a270722be7d7b61d77434a7f98c4276d6fc9694d0c86cacf03c72dc35ca17b5e7d3

diff --cc src/lib.rs
index 0779eec4eb81e3538287f398d9c80032d1bb8250,436db56f00953c33aa4255d845d8f95d2a61eed6..4ef971b717aef9134fe36c5620f1ca595e1990aa
--- 1/src/lib.rs
--- 2/src/lib.rs
+++ b/src/lib.rs
@@@ -44,18 -44,19 +44,19 @@@
  //! interact with the bitcoin P2P network.
  //!
  //! ```toml
 -//! bdk = "0.16.1"
 +//! bdk = "0.17.0"
  //! ```
+ //!
+ //! # Examples
  #![cfg_attr(
      feature = "electrum",
      doc = r##"
  ## Sync the balance of a descriptor
  
- ### Example
  ```no_run
 -use bdk::Wallet;
 +use bdk::{Wallet, SyncOptions};
  use bdk::database::MemoryDatabase;
 -use bdk::blockchain::{noop_progress, ElectrumBlockchain};
 +use bdk::blockchain::ElectrumBlockchain;
  use bdk::electrum_client::Client;
  
  fn main() -> Result<(), bdk::Error> {
@@@ -106,11 -106,10 +106,10 @@@
      doc = r##"
  ## Create a transaction
  
- ### Example
  ```no_run
 -use bdk::{FeeRate, Wallet};
 +use bdk::{FeeRate, Wallet, SyncOptions};
  use bdk::database::MemoryDatabase;
 -use bdk::blockchain::{noop_progress, ElectrumBlockchain};
 +use bdk::blockchain::ElectrumBlockchain;
  use bdk::electrum_client::Client;
  
  use bitcoin::consensus::serialize;