Instead of warning newcomers about pitfalls with nix-env, what if we just not mention it all

[zupo]

And some proof-read / local dev fixes.

[zupo]

Hey @ncfavier, thanks a ton for maintaining search.nixos.org, it's an absolute gem! I created this PR after hearing a bunch of people complain it's time we really stop showing people to use nix-env, so I went searching where we still do it, and found it here.

I'm perfectly happy removing any and all proof-read / local dev changes, just let me know which!

[SomeoneSerge]

I think we absolutely should remove nix-env -iA because it's fairly confusing and comes with many unintended consequences. However, we should probably direct users at least somewhere so that they may be able to manage "persistent" Nix package installations on FHS/non-NixOS systems: e.g. the home-manager, or (idk if it actually handles the use-case I think it handles) flox