Add basic usage docs for Build-Toolkit-Docs.ps1 script

# Background

Branched from https://github.com/CommunityToolkit/Tooling-Windows-Submodule/issues/280#issuecomment-2776951004

> It only works if you run from the components folder, maybe we should make it more robust so that it can run from the tooling and repo root as well?

The `Build-Toolkit-Docs.ps1` script was designed to be run from the root only. Running without arguments will output to the `./docs` folder in the repo root. From there, they're usually quick-inspected for issues and copied over to the official docs repo, or you can supply a custom `OutputDir` like you did here.

# Problem

The script was used in an unexpected way (not run from root), which led to unnecessary complexity and confusion when it had problems.

# Solution

Basic usage examples should be documented in the powershell script itself. It should be clear that this script, like all our other tooling build scripts, are designed to be run from the root of the repository.