add admonitions for macOS 15 Sequoia update

[abathur]

Same premise/language as NixOS/nix#11487

The impending release of macOS 15 Sequoia will break many existing nix installs on macOS, which may lead to an increased number of people who are looking to try to reinstall Nix without noticing the open/pinned issue (NixOS#10892) that explains the problem and outlines how to migrate existing installs.

These admonitions are a short-term measure until we are over the hump and support volumes dwindle.

[abathur]

Not immediately evident how to resolve this formatting block (#1489) from macOS (at least x86?):

$ nix run .#format
...
error: flake 'git+file:///Users/abathur/work/nixos-homepage' does not provide attribute 'apps.x86_64-darwin.format', 'packages.x86_64-darwin.format', 'legacyPackages.x86_64-darwin.format' or 'format'

$ nix-shell -p nodePackages.prettier
warning: Nix search path entry '/nix/var/nix/profiles/per-user/root/channels' does not exist, ignoring
this path will be fetched (0.89 MiB download, 8.05 MiB unpacked):
  /nix/store/81hh22bg003pw9ffp19idzlj11w8aj08-prettier-3.1.1
copying path '/nix/store/81hh22bg003pw9ffp19idzlj11w8aj08-prettier-3.1.1' from 'https://cache.nixos.org'...

$ prettier --write ./src
[error] Cannot find package 'prettier-plugin-astro' imported from /Users/abathur/work/nixos-homepage/noop.js

$ exit
$ nix-shell -p nodePackages.{prettier,prettier-plugin-astro}
...
       error: attribute 'prettier-plugin-astro' missing
...

[thilobillerbeck]

looks alright, thanks for your contribution <3