Add prerequisites section to README (#47)

## Summary
- Add a Prerequisites section listing Node.js >= 18, Yarn, and
PowerShell 7+ with install links
- Helps contributors avoid setup issues when running `.\build.ps1
-Bootstrap` for the first time

## Test plan
- [ ] Verify README renders correctly on GitHub

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Signed-off-by: Gilbert Sanchez <[email protected]>
Co-authored-by: Claude Opus 4.6 (1M context) <[email protected]>
Co-authored-by: Gilbert Sanchez <[email protected]>
Co-authored-by: Copilot <[email protected]>

Author: 4 people

README.md

@@ -6,6 +6,12 @@ This website is built using [Docusaurus](https://docusaurus.io/), a modern
 static website generator. The main feature for us was it's ability to
 support versioned docs.
 
+## Prerequisites
+
+- [Node.js](https://nodejs.org/) >= 18.0
+- [Yarn](https://classic.yarnpkg.com/) (`npm install -g yarn`)
+- [PowerShell 7+](https://learn.microsoft.com/en-us/powershell/scripting/install/installing-powershell) (run `pwsh`)
+
 ## Installation
 
 ```powershell