Add section-aware OG images for docs previews

[BrianSeong99]

Problem

The docs site currently uses one global social preview image for every page. When a specific docs URL is shared in Twitter/X, Telegram, Slack, or other unfurlers, the title and description may be page-specific, but the preview image does not communicate the page's section or context.

This makes shared docs links look generic and less useful, especially for index pages and important builder/core-concepts sections.

Proposed scope

Add a section-aware Open Graph / Twitter preview image system:

• Keep page title and description page-specific for SEO and link previews.
• Use dedicated OG images for major index pages and sections.
• If the shared page is an index page, show that index page's own preview image.
• If the shared page is a detailed child page, use the nearest parent/index/category preview image.
• Keep a global fallback image for pages that do not match a known section.
• Generate 1200x630 PNG assets so previews work reliably across Twitter/X, Telegram, Slack, Discord, and other unfurlers.
• Centralize routing logic so individual docs pages do not need manual image: front matter unless they need a special override.

Initial section image candidates

• Miden Docs / global fallback
• Build
• Get Started
• Smart Contracts
• Tutorials
• Tools
• Clients
• React SDK
• Rust Client
• Note Transport
• Miden Guardian
• Private Multisig
• Core Concepts
• Protocol
• Node
• Miden VM
• Compiler

Acceptance criteria

• Docusaurus emits page-specific title/description metadata unchanged.
• Docusaurus emits section-aware og:image / twitter:image metadata for docs pages.
• Explicit front matter image: still overrides the automatic section image.
• Generated images are committed under static/img/og/ or an equivalent static path.
• The visual design reads as Miden documentation, not just a logo pasted onto a static background.
• npm run build passes.
• At least a few built pages are checked for the expected metadata, including an index page and a detailed child page.