Add pipeline that's always green, for doc-only PRs

[wtgodbe]

Today, when you open a PR against any of the following paths, no CI job will run. Therefore, Build Analysis hangs indefinitely, because it's waiting for any pipeline to finish - this means devs can't merge such PRs without being an admin, or knowing the magic incantation to unblock Build Analysis (ba-g {justification}).

If we add a new "always green" pipeline that only runs on such PRs, we can work around the issue until dotnet/dnceng#5442 is resolved. Note that a github action wouldn't work here, as Build Analysis only looks for AzDO pipelines.

    - .devcontainer/*
    - .github/*
    - .vscode/*
    - docs/*
    - '**/*.md'
    - LICENSE.TXT
    - THIRD-PARTY-NOTICES.TXT

[Copilot]

Pull Request Overview

This PR adds a new pipeline (".azure/pipelines/report-green.yml") that always exits successfully for PRs affecting non-critical paths (such as docs, markdown files, and configuration directories).

• Introduces a pipeline triggered on PRs with specific path filters.
• Leverages an always-success job (using "exit 0") to unblock Build Analysis on doc-only PRs.