Document client authentication via client secret JWT

[maartenba]

Document client authentication via client secret JWT (see DuendeSoftware/products#2062)

Can probably go on https://docs.duendesoftware.com/identityserver/tokens/client-authentication/

Target releases/is-7.3 branch with your PR.

[wcabus]

After discussing client_secret_jwt, the behavior is going to change, so the docs update will focus on the migration guide instead

We might want a sentence or two (including that warning you mentioned) in the upgrade guide, because this is a behavior change. My instinct is to not do more than that unless this comes up in support.