docs: narzedzie na stronach, na ktore ludzie juz trafiaja - #219

Measured before writing anything: zero of the six error pages mentioned
zerosmtp-check, the main README mentioned it zero times, and one of nineteen
docs pages did. The tool answers the exact question those pages are about.

So somebody searches an error string, lands on the page written for that
string, reads several hundred words, and never learns there is a one-line
command that would have told them in two seconds. That is not a marketing gap.
It is a page describing a problem while withholding the thing that solves it.

The error pages are generated, so the block goes in the generator and all six
got it at once - along with every one added later, which is the point of
fixing it there rather than in the pages.

README: the blockquote about 535 5.7.139 already sat at the highest-intent
moment on the highest-traffic page - 46 of 55 unique visitors land on the
repository root - and pointed at a docs page. It now also gives the command.

TROUBLESHOOTING.md is the npm package's own homepage, and at the exact point
where it says "run the connectivity-only check" it offered ./check-connection.sh
- which requires cloning the repository first. That wording was correct when
the package returned 404. It has not been since Friday. npx goes first now and
the shell script stays for machines with no Node, which is a real case on a
locked-down Windows box.

PRINTERS.md gets the 1102 path, because a panel code is the hardest version of
this to act on: it looks like hardware failing and says nothing about a mail
server. The answer a technician needs on a service call is that the device does
not need replacing, and now the page says it.

Not added to the other fifteen docs pages. Repeating it everywhere would be
padding, and a reader who sees the same block on every page stops reading all
of them.