fix(hydra): show non-writeable identifier properties

[Spomky]

Q	A
Branch?	2.7 / 3.0
Tickets	[#4901)
License	MIT
Doc PR	api-platform/docs#...

Hi,

This is a proposal for fixing the bug in refernece. As I am not sure how to address it, I just start this PR.

I understand that the property should be listed in the documentation when it is marked as readable or writtable. Let me know if you are fine with it. I will try to propose a test to prevent potential regressions.

Regards.