Clean up Addons & Flyout menu docs (#11706)
* Clean up Addons & Flyout menu docs
This cleans up the copy here,
adds the DocDiff page,
and clarifies as bit more the Custom event language.
* Update docs/user/flyout-menu.rst
Co-authored-by: Manuel Kaufmann <humitos@gmail.com>
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Co-authored-by: Manuel Kaufmann <humitos@gmail.com>
