[#245] Use Open Graph images for all product and general posts

[vharseko]

Summary

Merge after #244 — 20 OpenIDM posts point at openidm-og.png, which #244 adds.

• Logo strip → OG card (28 posts) — imageurl switched from the ~700×172 *-logo.png to the matching 1024×512 *-og.png: OpenIDM 12, OpenDJ 9, OpenAM 6, OpenIG 1.
• Product posts without an image (21 posts) — each gets its product's card: the first entry of products:, or the product in the file name for posts without products: (OpenAM JEE agents, OpenIDM 7.0.1 / 7.1.0).
• General articles (6 posts) — a new assets/img/oip-og.png, built from oip-star.png on a white 1024×512 canvas, for stateless-vs-stateful-authentication, adaptive-authentication, passwordless-authentication-methods, sso-seamless-authentication-enterprise-client-services, llm-in-access-management and how-to-auth-via-esia.
• ESIA article title — landing-title / landing-title2 had the home page's "Welcome to Open Identity Platform Community" copied into them; they now use the article's own heading, "Аутентификация через госуслуги (ЕСИА)". With [#231] Fix page titles and empty og:image in header meta #239 this also becomes the page <title>.

Left to other PRs, as noted in the issue: the OpenICF posts (#244), the redirect … copy.md (#242) and the empty imageurl: '' (#239).

Fixes #245

Verification

Built the site with bundle exec jekyll build (Ruby 3.1, as in CI) and ran the workflow's htmlproofer internal-link check: no errors.

og:image across the built posts: openam-og.png 85, opendj-og.png 44, openig-og.png 24, openidm-og.png 20, oip-og.png 6. The rest are exactly the posts handled elsewhere: 10 openicf-logo.png + 3 GitHub avatar (OpenICF, #244) and 1 empty value (#239). On this branch alone openidm-og.png does not exist yet — hence the merge order above.

git merge-tree against the branches of #239–#244: no conflicts.

Preview

[maximthomas]

LGTM