Add LearningResource structured data to curated resources

## Why this needs addressing

Individual curated-resource pages already expose structured front-matter fields such as abstract, material type, education level, primary user, subject area, language, creators, and the external resource URL. However, the live page `https://forrt.org/curated_resources/stereotype-threat/` currently exposes BreadcrumbList structured data but no content entity describing the resource.

## Requested change

Emit a `LearningResource` JSON-LD entity from `layouts/curated_resources/single.html` (or a dedicated partial), populated from existing front matter. Where appropriate include:

- `name`, `url`, and `abstract`;
- `learningResourceType`, `educationalLevel`, and `audience`;
- `about`, `inLanguage`, and `dateModified`;
- verified creators/authors;
- `isBasedOn` for `link_to_resource`;
- the FORRT provider/organization.

Use the actual values and omit fields that are absent or ambiguous. Do not claim that generic `LearningResource` markup guarantees a Google rich result; the goal is accurate machine-readable semantics.

## Acceptance criteria

- Representative resource pages emit valid JSON-LD with no empty arrays or placeholder values.
- The structured data matches the visible page content and front matter.
- Existing BreadcrumbList and site-wide Organization data remain valid.
- Validate the output with a JSON parser and a Schema.org validator.

Reference: https://schema.org/LearningResource