[Website] Add "Opt-In Features" blog post

[glen-84]

Summary

• Adds a blog post on opt-in features and the @requiresOptIn directive: its Kotlin origins and the Stage 0 GraphQL RFC, how it pairs with @deprecated to complete the feature lifecycle, and how to use it in Hot Chocolate and Fusion.
• Covers all five directive locations, the includeOptIn, requiresOptIn, optInFeatures, and optInFeatureStability introspection surface, stability levels, and Fusion's union merge and OPT_IN_FEATURE_STABILITY_MISMATCH rule.

The frontmatter references a header.png that is not part of this PR yet. The image needs to be added under website/public/images/blog/2026-09-08-opt-in-features/ before the post is published.

[Copilot]

🟡 Changes recommended

The post references a featuredImage (header.png) that is not present under the expected public blog image directory, which will render a broken header image.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

Adds a new website blog post explaining GraphQL opt-in features via the @requiresOptIn directive, including background, usage in Hot Chocolate, and behavior in Fusion composition/introspection.

Changes:

• Adds a new blog post covering the @requiresOptIn directive and the opt-in feature lifecycle alongside @deprecated.
• Documents usage patterns (implementation-first, code-first, schema-first) plus introspection and stability metadata.
• Explains Fusion composition semantics and the OPT_IN_FEATURE_STABILITY_MISMATCH rule.

File summaries

File	Description
website/content/blog/2026-09-08-opt-in-features.md	New blog post content and frontmatter for the “Opt-In Features” article.

Review details

• Files reviewed: 1/1 changed files
• Comments generated: 1
• Review effort level: Lite

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