Update templating docs: dotnet/templating absorbed into dotnet/sdk

[mthalman]

Why

The dotnet/templating repository has been absorbed into dotnet/sdk: the template engine library (Microsoft.TemplateEngine.*) lives in this repo. Several docs still instructed contributors to route templating changes and issues to the now-defunct external repo, which is misleading.

What changed

• .github/copilot-instructions.md: Removed the ## External Dependencies section that told agents to make templating changes in the external dotnet/templating repo and file issues there.
• template_feed/Microsoft.TemplateEngine.Authoring.Templates/README.md: Feedback/issue link now points to the dotnet/sdk repo.
• documentation/TemplateEngine/api/Inside-the-Template-Engine.md: Reworded the host description to "The .NET SDK includes one such host: the dotnet new CLI."
• documentation/TemplateEngine/authoring-tools/Localization.md: "each build of dotnet/templating repo" now reads "each build of the dotnet/sdk repo."

Notes

Docs-only change. documentation/TemplateEngine/README.md already describes the merge correctly and was left untouched. Historical issue-tracker links of the form https://github.com/dotnet/templating/issues/NNNN were intentionally left as-is since they point to real archived issues.

[Copilot]

Pull request overview

Updates contributor-facing documentation to reflect that template engine sources and dotnet new now live in dotnet/sdk, removing outdated guidance that redirected work to the former dotnet/templating repo.

Changes:

• Remove Copilot agent guidance that treated dotnet/templating as an external dependency.
• Update templating docs/README wording to reference dotnet/sdk as the active repository/host context.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
.github/copilot-instructions.md	Removes obsolete “External Dependencies” section that redirected templating changes to dotnet/templating.
template_feed/Microsoft.TemplateEngine.Authoring.Templates/README.md	Updates contribution/feedback wording to point to dotnet/sdk instead of dotnet/templating.
documentation/TemplateEngine/api/Inside-the-Template-Engine.md	Rewords host description to reflect dotnet new being included with the .NET SDK.
documentation/TemplateEngine/authoring-tools/Localization.md	Updates localization guidance to reference builds of dotnet/sdk rather than dotnet/templating.