Document end-of-life dates

[bnoordhuis]

Apologies if this has been discussed before but searching npm/documentation and npm/cli didn't turn up anything.

The website and the CLI docs don't seem to mention end-of-life dates for npm's release lines. Would it be possible to add those?

[lukekarrys]

We do have a documented policy for CLI support here: https://github.com/npm/cli/wiki/Support-Policy

But I agree this would be much better served if it were available (and canonical) somewhere on the docs site.