Consolidate infra artifacts under repo root or eng folder

[MarcoRossignoli]

At the end of all "refactor" of "builds" we should decide if move all "files"(yaml and scripts) under eng[ineering] folder or simply in root repo.

cc: @tonerdo @AArnott

[AArnott]

I'm a fan of keeping relatively few files in the root of the repo. In fact I keep a src folder in each of my repos where all solution-related assets reside. I leave the root of the repo for READMEs and other common interest files. I won't push that too hard here though, since I might be moving version.json and global.json back to the root of my repos for practical convenience reasons.

I usually keep my azure-pipelines.yml file in the root, since that's where Azure Pipelines will find it automatically. Then the templates that it imports come from an azure-pipelines folder under the root.
If you have a build script, that's probably most convenient/discoverable if left in the root of the repo as well, but if it brought in many more files, hiding those in some sub-folder sounds good to me.