Docs: Simplify the changelog of 4.8.0 and move the auth changes to Unreleased

[lehni]

• The auth PRs (Server: Replace passport with Dito.js' own login and session handling #326, Server, Admin: Add passkey authentication #340, Server, Admin: Add two-factor authentication with TOTP #342, Server, Admin: Add password reset #346) were merged after 4.8.0 was released, and their entries landed in the 4.8.0 section. They move to a new Unreleased section, as they're not in 4.8.0.
• The entries of 4.8.0 and Unreleased are shortened to what changes and what to do, with the details left to the linked PRs and docs.