Update Async Task documentation

## How can Bevy's documentation be improved?

The [current examples](https://github.com/bevyengine/bevy/blob/a291b5aaedb4affcb31df2e2e63cb0c665ffb24a/examples/async_tasks/async_compute.rs) for using async computing are out of date. Specifically, they use the [older v0.7 syntax](https://bevyengine.org/learn/migration-guides/0.7-0.8/#task-doesn-t-impl-component) of `Res`, which will not compile in current Bevy (0.11.2).

Additionally, the async compute and `Task` system is not highlighted at all in current documentation. Coming from Unity's DOTS and `Job` pipelines, I was looking for a similar analog.

As luck would have it, I ran across [Logic Projects' great video](https://www.youtube.com/watch?v=QTUEyAZmdv4) on Pathfinding + Bevy Async Tasks which was exactly what I was trying to do. The relevant source code can be found on the [Github project](https://github.com/mwbryant/logic_management_game/blob/master/src/pathfinding.rs).

I think it would be immensely useful for developers to know that Bevy supports this, and refreshed documentation on how to use it.

Start with examples/async_tasks/async_compute.rs and compare its Res usage with the 0.7–0.8 migration guide and the current Bevy API. Review the linked pathfinding source for a current usage example. Done means the async-compute examples compile with Bevy 0.11.2 and the documentation highlights the async compute and Task system.

Written by the indexing model from the issue text.