Update docs for non-nix compat to include vim.pack.add

[BirdeeHub]

use this for context on the old one

and check the vim.pack.add example here

update all docs and other templates to use that where applicable

maybe delete this from nixCatsUtils because vim.pack.add is a thing now? Its just a template, you can keep using it if you want

All of this is only relevant if you try to use your config built via nixCats WITHOUT ANY NIX whatsoever, and changing templates and docs will not change your config, it will at most change the content of :h nixCats.*