Fix outdated @flattenProperty link in no-openapi documentation (#3579)

The `@flattenProperty` decorator link in the no-openapi rule
documentation points to a non-existent anchor. The decorator was moved
to the `Azure.ClientGenerator.Core.Legacy` namespace but the link was
not updated.

- Updated anchor from `#@Azure.ClientGenerator.Core.flattenProperty` to
`#@Azure.ClientGenerator.Core.Legacy.flattenProperty`

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>[doc] no-openapi document is out of date</issue_title>
> <issue_description>The link of @flattenProperty is outdated in this
document:
https://azure.github.io/typespec-azure/docs/libraries/azure-core/rules/no-openapi/.</issue_description>
> 
> <agent_instructions>Change the link to @flattenProperty to the actual
jsdoc link in Azure.Core.Legacy namespace</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>

- Fixes #3575

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Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: markcowl <[email protected]>

Author: Copilot

website/src/content/docs/docs/libraries/azure-core/rules/no-openapi.md

@@ -17,7 +17,7 @@ Those decorators are only meant to be read by the openapi emitters which means t
 | ------------------------------------ | --------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `@example`                           | [See examples doc](../../../migrate-swagger/faq/x-ms-examples.mdx)                                                                                              |
 | `@extension("x-ms-examples", `       | [See examples doc](../../../migrate-swagger/faq/x-ms-examples.mdx)                                                                                              |
-| `@extension("x-ms-client-flatten", ` | Use [`@flattenProperty`](../../typespec-client-generator-core/reference/decorators.md#@Azure.ClientGenerator.Core.flattenProperty)                              |
+| `@extension("x-ms-client-flatten", ` | Use [`@flattenProperty`](../../typespec-client-generator-core/reference/decorators.md#@Azure.ClientGenerator.Core.Legacy.flattenProperty)                       |
 | `@extension("x-ms-mutability", `     | Use [`@visibility` decorator](https://typespec.io/docs/standard-library/built-in-decorators#@visibility)                                                        |
 | `@extension("x-ms-enum", `           | [Enum extensibility doc](../../../troubleshoot/enum-not-extensible.md)                                                                                          |
 | `@extension("x-ms-identifiers", `    | Use [`@identifiers`](../../azure-resource-manager/reference/decorators.md#@Azure.ResourceManager.identifiers)                                                   |