docs: no-required-schema-properties-undefined markdown docs (#1417)

docs/rules/no-required-schema-properties-undefined.md

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+---
+slug: /docs/cli/rules/no-required-schema-properties-undefined
+---
+
+# no-required-schema-properties-undefined
+
+Ensures there are no required schema properties that are undefined.
+
+| OAS | Compatibility |
+| --- | ------------- |
+| 2.0 | ✅            |
+| 3.0 | ✅            |
+| 3.1 | ✅            |
+
+```mermaid
+flowchart TD
+
+Root ==> Components ==> Schemas
+
+style Schemas fill:#codaf9,stroke:#0044d4,stroke-width:5px
+```
+
+## API design principles
+
+If a required schema property is declared but not defined, this rule informs you which of the required schema properties are missing.
+
+## Configuration
+
+| Option   | Type   | Description                                                                              |
+| -------- | ------ | ---------------------------------------------------------------------------------------- |
+| severity | string | Possible values: `off`, `warn`, `error`. Default `off` (in `recommended` configuration). |
+
+An example configuration:
+
+```yaml
+rules:
+  no-required-schema-properties-undefined: error
+```
+
+## Examples
+
+Given this configuration:
+
+```yaml
+rules:
+  no-required-schema-properties-undefined: error
+```
+
+Example of **incorrect** schema properties:
+
+```yaml
+schemas:
+  Pet:
+    type: object
+    required:
+        - id
+        - name
+    properties:
+        id:
+          type: integer
+          format: int64
+```
+
+Expected error message when linting incorrect schema example:
+
+```bash
+Required property 'name' is undefined.
+```
+
+Example of **correct** schema properties:
+
+```yaml
+schemas:
+  Pet:
+    type: object
+    required:
+        - id
+        - name
+    properties:
+        id:
+          type: integer
+          format: int64
+        name:
+          type: string
+          example: doggie
+```
+
+## Related rules
+
+- [no-invalid-schema-examples](./no-invalid-schema-examples.md)
+
+## Resources
+
+- [Rule source](https://github.com/Redocly/redocly-cli/blob/main/packages/core/src/rules/common/no-required-schema-properties-undefined.ts)
+- [Schema docs](https://redocly.com/docs/openapi-visual-reference/schemas/)