Add section-aware docs OG images

[BrianSeong99]

Summary

Adds section-aware Open Graph / Twitter preview images for docs pages so shared links keep page-specific title/description metadata while showing a useful section image instead of the same global fallback everywhere.

Closes #294.

What changed

• Adds a central route-to-OG-image mapper for current, /next, and versioned docs paths.
• Overrides DocItem/Metadata so normal docs pages use explicit frontmatter images when present, otherwise inherit the nearest section image.
• Overrides DocCategoryGeneratedIndexPage so generated category pages also get section images.
• Adds 39 generated 1200x630 PNG assets under static/img/og/.
• Adds npm run generate:og to regenerate the image set from a single script.
• Updates the global site fallback image to img/og/miden-docs.png.

Verification

• npm run generate:og
• git diff --check
• Verified every mapped image exists under static/img/og/
• NODE_OPTIONS=--max-old-space-size=12288 npm run build after local deploy-style ingestion of external docs
• Checked built metadata for:
  • /next/builder/tools/clients/react-sdk/query-hooks/ → page title/description + react-sdk.png
  • /builder/tools/clients/react-sdk/query-hooks/ → page title/description + react-sdk.png
  • /builder/smart-contracts/accounts/ → generated category title + smart-contract-accounts.png
  • /next/builder/tools/note-transport/users/ → page title/description + note-transport.png

Notes

The full build still reports existing broken-link, broken-anchor, KaTeX, and Browserslist warnings unrelated to this change. npm run typecheck also still reports pre-existing Docusaurus alias/type errors in swizzled and versioned theme files; the new files do not appear in the filtered typecheck output.