Update Tor Browser manual for 8.5 release about Security Settings

The TB 8.5 security settings have a different functionality of previous versions. We should update the documentation content to reflect the new behavior.

As described in #29657 (moved), the link should be https://tb-manual.torproject.org/en-US/security-settings/ instead of /security-slider.

The content is being discussed here: https://pad.riseup.net/p/ZsGRD4yM5pshKddf75IR-keep