Update documentation for nixpkgs breaking change

[importantblimp]

Summary

While doing due diligence for https://github.com/nixos/nixpkgs/issues/180654 I noticed this repository has documentation for enabling Kime on NixOS. The fix for 180654 is a breaking change, so this PR updates the Kime documentation.

I haven't updated the changelog because this is a NixOS breaking change, and is documented in the NixOS breaking changes for release 24.11. Existing non-NixOS users are unaffected, and existing NixOS users will be informed when they update to 24.11. New users in both cases are unaffected.

Checklist

• [x] I have documented my changes properly to adequate places
• [x] I have updated the docs/CHANGELOG.md

[importantblimp]

The nixpkgs review has been marked as a draft, pending the end of the breaking changes window and some changes to the PR

[importantblimp]

The change is now present in https://github.com/NixOS/nixpkgs/commit/9665639708ae46177c4b71a3550cf43d577c6c46 and, although the change is no longer breaking, enabled has been deprecated and will be removed in a future release