Add Spec Validation (#117)

* Add first version of spec validation article.

* Update validate-spec.md

* Rename validate-spec -> openapi-validation. Update OAS -> OpenAPI. Add callout regarding v3 support. Add related section at bottom.

* use a blockquote + latest markdown annotation idea

articles/modeling/openapi-validation.md

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+# OpenAPI Validation
+
+![](../../assets/gifs/file-validation-oas-spec.gif)
+
+OpenAPI validation is the process of verifying the underlying OpenAPI file syntax by making sure it conforms to the [OpenAPI Specification requirements](https://github.com/OAI/OpenAPI-Specification#the-openapi-specification) provided by the [OpenAPI Initiative](https://www.openapis.org/). Stoplight immediately validates any changes done to a spec to ensure they are in the correct format prior to being saved.
+
+<!-- theme: info -->
+> Stoplight currently supports the OpenAPI v2 specification. We are working on support for OpenAPI v3, and should have more information in the coming months.
+
+***
+
+**Related**
+
+* [File Validation](../editor/file-validation.md)

articles/modeling/validate-spec.md

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-