Determine GitHub Copilot prompt guidance in monorepo

[mccoyp]

@jayesh-tanna opened a sample PR (#44956) to demonstrate how services -- ML, in this case -- might want to define Copilot prompts. Since we have a monorepo with many services, it's important that these prompts are organized in a scalable, usable way that preserves functionality while avoiding cluttered usage, e.g. when specifying a prompt in a GitHub Copilot chat window.

When reviewing the images in the GitHub Copilot chat window, you’ll notice that pressing / displays all prompts. Given that the azure-sdk-for-python repository contains hundreds of libraries, this could result in a large number of prompts appearing when the user presses /. Similarly, the agents created by each library will appear in the agent popup in the same order. We can store instructions and skills within the appropriate subdirectory (e.g., .github/instructions/azure-ai-ml/someinstruction.md), while prompts and agents can be placed in the root directory (e.g., .github/agents/some_agent.md).
Proposal for this issue:

1. We can distinguish prompts by name, as demonstrated in this pull request.
2. Consider having common prompts, skills, or agents across libraries, as there are already some prompts and instructions in the .github folder.