[AZ SDK MCP docs] copilot cli requirement for standalone azsdkcli executable

In the eng hub docs we should include the following information in some form:

When running azsdkcli as a standalone executable (not via dotnet tool), the GitHub Copilot CLI executable must be
available for copilot-dependent features (customized code update, sample generation, sample translation, readme
generation).

The GitHub.Copilot.SDK NuGet package bundles the binary automatically during publish, but we cannot take advantage of that in our release pipeline because of some kind of permissions issue.

When the executable isn't found, users can set AZSDK_COPILOT_CLI_PATH to point to an existing installation.

What to document:
- Which features require the Copilot CLI
- How the SDK resolves the executable path (bundled → env var override)
- The AZSDK_COPILOT_CLI_PATH environment variable as a fallback
- The improved error message now points users to this workaround

Context: Azure/azure-sdk-tools#15276

Start in the eng hub documentation for the standalone azsdkcli executable and use the requirements listed in this issue, with Azure/azure-sdk-tools#15276 for context. Document which features require the GitHub Copilot CLI, the bundled-to-environment-variable resolution order, AZSDK_COPILOT_CLI_PATH, and the improved missing-executable error; done when users can follow the workaround.

Written by the indexing model from the issue text.