Add v8 changes from products repo and include IdentityServer4 to v8 upgrade guide (#1080)

* Add changes from products repository (PR 255 and 256), optimize for readability in docs site

* Reorganize IdentityServer upgrade guide overview for improved clarity and consistency

* Add IdentityServer4 to Duende IdentityServer v8.0 upgrade guide detailing migration steps, breaking changes, and schema updates.

Author: maartenba