Improving documentation structure

As Hamilton and its documentation are growing, the documentation requires additional structure. Here are a few tasks to improve the documentation, please comment on this issue if you're interested in undertaking one!

1. The `Integrations` section could be refactored to "Plugins" (i.e., libraries that Hamilton has extensions for) and "Cookbook" (i.e., examples on how to use FastAPI and Hamilton)
2. The API reference section should have a section about plugins
3. The `Lifecycle Adapters` section should separate the lifecycle API (i.e., GraphExecutionHook, NodeExecutionMethod, etc.) from the actual adapter implementations (SlackNotifier, ProgressBar)