docs: nowa akcja tam, gdzie ludzie czytaja, i sprostowanie mojego twierdzenia - #229

The Marketplace listing for ZeroSMTP Check went live today. Neither the README
nor any of the nineteen docs pages mentioned it, which is the same failure as
this morning: a tool shipped and the pages people actually read said nothing
about it.

Correcting something I said while building it. I described the Marketplace as
the one GitHub surface this project was absent from. That was wrong and one
request would have checked it - msgwing/send-email-action has been listed at
marketplace/actions/zerosmtp-send-email since early August. The claim was made
from memory of what we had built rather than from looking.

What the check does confirm is more useful than the claim was: that listing has
zero stars and has not moved since 2026-08-02. Presence on the Marketplace was
already there and produced nothing measurable, so the second listing should not
be expected to do much on its own either. It is a shelf, not a campaign.

The README section now names both and says which is which, because they do
opposite things and one line describing "our GitHub Action" would leave a
reader guessing: one checks that mail can be sent, the other sends it.

MONITORING.md gets certificate expiry as its own section. Every check on that
page watches whether the relay answers. None watched the failure that arrives
with no warning at all - nothing degrades, no threshold is crossed, it works on
Saturday and on Sunday it does not.