Add a release post JSON schema

Why is this change being made?

JSON schema is the most popular schema format for JSON and YAML files. Adding the schema file allows it to be used in VSCode and perhaps in the future by the GitLab Web IDE.

This change does not replace the existing linting and validation of release post items, but can be considered preparation for some refactoring.

To use this locally in VS Code while working on release post items:

1. Install https://marketplace.visualstudio.com/items?itemName=redhat.vscode-yaml
2. Update your VSCode workspace config (see https://code.visualstudio.com/docs/getstarted/settings)

{
    "yaml.schemas": {
        "data/schemas/releasepost.schema.json": ["data/release_posts/*/*.yml", "data/release_posts/*/*.yaml"],
    }
}

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Author Checklist

• Correct MR template applied (e.g blog post)
• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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