docs: getting started documentation for web FirebaseUI v7

[russellwheatley]

• Wrote a getting started document for the latest FirebaseUI web library to mirror closely the previous: https://firebase.google.com/docs/auth/web/firebaseui

[gemini-code-assist]

Summary of Changes

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This pull request adds a comprehensive getting started guide for the latest FirebaseUI for Web library. It provides developers with step-by-step instructions on how to integrate FirebaseUI into their web applications, covering installation, initialization, authentication method setup, and UI customization for React, Shadcn, and Angular platforms. The guide also highlights key differences from the previous FirebaseUI version and offers migration guidance.

Highlights

• Getting Started Guide: This PR introduces a new 'Getting Started' document for the latest FirebaseUI web library, mirroring the structure of the previous version.
• Modernization: The new FirebaseUI for Web features a complete rewrite with a modern modular architecture, offering benefits like modular SDK support, composable UI elements, and support for React, Shadcn, and Angular.
• Configuration: The guide details how to initialize FirebaseUI with the new initializeUI(...) method, set up sign-in methods, include styles, and configure behaviors for OAuth flows, Google One Tap, and anonymous user upgrades.

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[gemini-code-assist]

Code Review

This pull request introduces a comprehensive "Getting Started" guide that mirrors the structure of the classic FirebaseUI documentation. The guide is well-written and covers installation and usage for React, Shadcn, and Angular, along with key features like authentication methods, UI customization, and behaviors. I've identified a couple of minor inconsistencies in the documentation that should be addressed to ensure clarity for developers. My detailed feedback is in the comments below.

[gemini-code-assist]

The Shadcn registry URL provided here is inconsistent with what's mentioned in other documentation files, such as MIGRATION.md. To avoid confusion for users, it's important to use a consistent URL across all documents. I'd recommend double-checking which URL is the correct public one and updating all instances accordingly.

Suggested change

	{
	"registries": {
	"@firebase": "https://firebaseopensource.com/r/{name}.json"
	}
	}
	{
	"registries": {
	"@firebase": "https://fir-ui-shadcn-registry.web.app/r/{name}.json"
	}
	}