Add method of disabling docx abstract title

[LordAro]

Describe your proposed improvement and the problem it solves.

The "problem" is that the Abstract title appears whereas previously it did not.

Since #8702 / 80e750a , the docx output now always includes the abstract title 'Abstract'.

I'd quite like this to not be here and for there to be a way to disable it.

(In my actual document, subtitle is also used for something else so is not feasible to be (ab)used for this purpose)

Describe alternatives you've considered.

As suggested in #8702 (comment) HTML & epub have a abstract-title attribute, it'd be great if something like that could be used to disable it entirely.

Unless of course I'm missing something and there is in fact a way of disabling it?

[zero0n3]

Hello, could you please indicate here and/or in the documentation, how to use abstract-title as it is not clear how to make it work.
Thank you