Write release notes for PR #11906: Run the multi-threaded executor at the end of each system task.

[alice-i-cecile]

bevyengine/bevy#11906 needs release notes for the upcoming Bevy release!

Original authors: @chescock, @hymm, @james7132

Please reply below if you'd like to write these notes.
While the author(s) of the PR often have the context, knowledge and motivation to draft the release notes for their feature,
anyone can contribute release notes!

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Release notes should:

1. Clearly motivate the change.
2. Be written in a way that is understandable by the average Bevy user: some programming background and a general understanding of games.
3. Show off the coolest features of the PR. Screenshots are awesome, but elegant APIs are also welcome!
4. If this was a perf-centric PR, quantify the performance improvements. Graphs and statistics work well for this.

We can help you revise the release notes: a rough draft alone is incredibly useful :)
Your expertise is invaluable for contextualizing the changes; we'll work with you to bring the technical writing up to par.

To submit your release notes, modify ..\release-content\0.14\release-notes\11906_Run_the_multithreaded_executor_at_the_end_of_each_system_t.md and submit a PR.
In that PR, please mention this issue with the Fixes #ISSUE_NUMBER keyphrase so it gets closed automatically.

[alice-i-cecile]

The work done in bevyengine/bevy#12936 should probably be rolled into the same section, talking about how we've reduced system overhead across the board.

[hymm]

I can write something for this. Will try to get it done tonight or tomorrow.