feat: implement full OpenGraph support and update social image to match new brand

[vmorps]

Overview

During a review of the site's metadata, we identified that OpenGraph support is only partially implemented. Several critical tags are missing, and the existing social share image (opentdf-social.png) does not reflect the new brand design introduced in the homepage redesign.

This issue tracks the work to fully implement OpenGraph, add Twitter/X Card support, and replace the social image.

Current state

Docusaurus themeConfig.image is set to img/opentdf-social.png, which provides a global og:image fallback. The homepage has explicit og:title and og:description tags. Everything else is missing or incomplete.

Tasks

High priority

• Add og:url to the homepage (https://opentdf.io)
• Add og:type: "website" to the homepage
• Add og:image explicitly to the homepage (do not rely solely on the global fallback)
• Add og:image:width and og:image:height (standard: 1200×630)
• Add Twitter/X Card tags globally or on the homepage: twitter:card, twitter:title, twitter:description, twitter:image

Medium priority

• Add og:site_name: "OpenTDF" globally in docusaurus.config.ts
• Add og:image:width / og:image:height to the global headTags

Design

• Create a new opentdf-social.png (1200×630px) that reflects the new brand: dark background (#0a0c10), new OpenTDF icon, headline in cyan-to-purple gradient treatment, opentdf.io URL

Minor

• Fix Schema.org JSON-LD inconsistency: the logo URL currently points to https://docs.opentdf.io/img/opentdf-social.png but the org url is https://opentdf.io — update the logo URL to match once the new social image is in place

Acceptance criteria

• Pasting any opentdf.io URL into the LinkedIn post inspector, Twitter Card Validator, or Facebook Sharing Debugger returns a correctly populated preview with the new brand social image
• og:title, og:description, og:image, og:url, og:type, and og:site_name are all present on the homepage
• Twitter/X Card tags are present and valid
• Social image visually matches the new site design

🤖 Generated with Claude Code

[vmorps]

All tasks completed in PR #191 (feat/site-redesign):

High priority (previously done):

• og:url, og:type, og:image on homepage
• og:image:width / og:image:height
• Twitter/X Card tags

Medium priority (done in commit c793331):

• og:site_name: "OpenTDF" added globally via headTags
• og:image:width (1200) / og:image:height (630) added globally

Design (done in commit c793331):

• New opentdf-social.png — clean brand-focused design with lockup, gradient headline, and opentdf.io URL on dark background

Minor (done in commit c793331):

• Schema.org JSON-LD logo URL updated to https://opentdf.io/img/opentdf-social.png