Remark in articles that rely on INCLUDES file versioning

@cmastr @tdykstra @wadepickett

Per @tdykstra's remark at https://github.com/dotnet/AspNetCore.Docs/pull/36616#issuecomment-3795706652 on the *Middleware* article overhaul with ✨ *Blazorfication*™ ✨😄 ...

This is an issue to consider adding a remark (invisible HTML comment) at the tops of markdown files that rely on the INCLUDES file versioning approach telling readers that versioned content for prior releases resides in one or more separate files and that each file should be updated when common content is addressed by a PR.

Personally, I think it would be better to abandon the INCLUDES file versioning approach 💀, but that was only my call in the Blazor node, where no such approach was ever adopted in the first place.

Anywho ... if it's decided not to add such a remark at the tops of markdown files, this issue can be closed.