Document the use of a new password policy in theme (frontend)

[kpodemski]

https://github.com/PrestaShop/PrestaShop/pull/28127/

Base on: https://github.com/PrestaShop/classic-theme/pull/21

[matks]

Assigned to @NeOMakinG 😉

[matks]

@kpodemski I think we need to document 'how to use it in a theme' AND 'how it works'

[NeOMakinG]

@matks the backend part also needs to be documented and I won't be able to do it!

[matks]

@NeOMakinG Yes, I think we need to create a 2nd issue for the backend