Async-graph-data-flow

Added
- Python 3.12 is officially supported and tested on CI.
- Added a technical notes page to the Sphinx documentation.

Added
- `AsyncGraph.add_node` has the new keyword argument `check_async_gen`
to optionally disable the async generator function check for advanced usage.

Added
- An `AsyncExecutor` instance now has the `start_nodes` property.
It maps start nodes to their arguments passed in at the beginning of graph execution.

Fixed
- Before a graph executes, the `exceptions` property of an `AsyncExecutor` instance
now returns `None`. Previously, accessing this property before graph execute
would raise an exception.

Added
- Keep track of and expose exceptions. (6)

First release!