docs: add Marketplace status and release verification

[transitrix]

What changed

Adds a dated Marketplace status and release-verification page, linked from the installation guide. It distinguishes the four Open VSX 3.2.0 packages examined in the historical audit from unavailable Microsoft-served packages and from release 3.8.1. It links the public status update, audit finding, scans, rebuild comparison and release materials, and asks readers to keep security protections enabled.

The page uses the existing site layout, with a manifest declaration and regenerated sitemap. No deployment configuration changes.

How it is verified

• Existing indexing/reachability and OG/Twitter checks pass locally, including their self-tests; git diff --check passes.
• The new page matches the canonical icon declarations; favicon PNG alpha and manifest checks pass. The full icon workflow uses GNU mktemp --suffix, unavailable on the local Mac; GitHub CI will run that check on Linux.
• Public sources were read and linked. Release 3.8.1 assets and verification instructions were checked.
• GitHub Pages is configured to publish only main at /; no external repository webhooks or PR preview deployment workflow were found. This PR does not publish the page. No merge or manual deployment is requested.