[Docs] Hook up C++ SDK repo for docs issues

Customers visiting the C++ docs site should be able to provide feedback to the C++ SDK team directly (same as other languages):

Other docs sites have feedback links at the bottom for customers to open issues which can be directed to the SDK teams:

This information is configured in docfx.json through some set of values like this: https://github.com/MicrosoftDocs/azure-docs-sdk-python/blob/main/docfx.json#L157-L159

```
"feedback_system": "OpenSource",
"feedback_github_repo": "Azure/azure-sdk-for-python",
"feedback_product_url": "https://github.com/Azure/azure-sdk-for-python/issues",
```