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

[Arlodotexe]

Background

Branched from #280 (comment)

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.