docs: przybysze z Marketplace dostaja to, po co przyszli, nad zagieciem - #258

Checked the published listing rather than assuming it was fine. Title and
description are right, and GitHub generates the `uses:` snippet by itself. What
it shows as the body is this repository's README, which opens with "Everything
you need for the Microsoft 365 SMTP AUTH shutdown" - a mail relay pitch.

So a developer browsing Marketplace for a CI action lands on a product page for
something else, and has to scroll past a compatibility list, twenty-one code
examples and a star history chart before finding out what the action does.

A short block near the top now answers that in place: the snippet, what it
checks, the certificate window that is the actual reason to add it to a
pipeline, and a link to the full inputs further down. It is addressed to people
who arrived from the Marketplace so it does not read as noise to somebody who
came here for the relay.

One thing this does not fix and cannot from here: the listing is in no
Marketplace category at all, so nobody browsing by category can reach it. That
is set on the listing itself rather than in action.yml.