Document `book_changes` APIs

[mDuo13]

Documents the book_changes API method and subscription stream, which were added to rippled a few versions ago by https://github.com/XRPLF/rippled/pull/4212

• [x] Add book_changes API method docs
• [x] Add book_changes subscription stream docs
• [x] Add websocket tool example for book_changes method
• [x] Remove extraneous numerical IDs from other WebSocket tool examples

Not in scope:

• 2617

[mDuo13]

@tequdev Good catches, thanks!