Add copilot-instructions to repo

[wadepickett]

Fixes #35798

When we ask copilot to create a PR based on an issue, or to review a PR on this repo, copilot will use the instructions in this copilot-instructions.md to guide it and to facilitate workflow interaction with human authors or reviewers. The goal of the instructions is to help ensure Copilot's responses maintain consistent quality and follow established documentation standards and some minimal expected human Intractive workflow when assisting with ASP.NET Core documentation.

There isn't a way to preview or test the instructions locally before publishing them to the repository that I know of. This is because the instructions are processed by GitHub Copilot's systems when they're accessed from the repository. However, this copilot-intruction.md can be pointed to as a link in any GitHub Copilot session and then also point to an example issue or PR to see how it would react.

This was created based on the work of others for aspire and .net docs. (Thank you, David, Bill, Geneveive and Andy!):
https://github.com/dotnet/docs-aspire/blob/main/.github/copilot-instructions.md
https://github.com/dotnet/docs/blob/main/.github/copilot-instructions.md

I removed any duplication to the style guide and called out specifics needed for this aspnetcore.docs repo.

Also helpful:
https://docs.github.com/en/copilot/how-tos/configure-custom-instructions/add-repository-instructions?tool=vscode

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Internal previews

📄 File	🔗 Preview link
.github/copilot-instructions.md	.github/copilot-instructions

[wadepickett]

@danroth27, @tdykstra, @guardrex, @IEvangelist: I could use your thoughts on this early draft for a copilot-instructions.md specific to this aspnetcore.docs repo.

[guardrex]

Given the length, it's best if I can look early next week.

[danroth27]

Do we know that all of this content adds value? I'd prefer to start small with things that we know Copilot needs additional context on and then we grow the content organically as we run into issues. Otherwise, how do we know we're not just wasting tokens?

[wadepickett]

Thanks for the feedback:
Slimmed it waaaaay down. There was duplication with the style guide that should have been removed, thanks. Anything else in there are specific to this repo or helps with the interaction process.

[wadepickett]

Given the length, it's best if I can look early next week.

Thanks, It has been greatly reduced now already after feedback. haha. :)

[BillWagner]

This LGTM.

[wadepickett]

@danroth27 and @tdykstra, I will go ahead and merge this copilot-instructions.md at the end of the day today unless I see any more ideas or requests for change.

Approval from Gewarren, Bill and guardrex so far. Everyone had great feedback and ideas.

[tdykstra]

A Bit more to come but I'm Sending what I have now to get in under the EOD deadline