chore: agent skill for helping users setup with FirebaseUI Auth

[russellwheatley]

• Getting started skill to help consumers get started with FirebaseUI with react, angular or shadcn.
• Followed skill best practices: agentskills.io/skill-creation/best-practices

[gemini-code-assist]

Code Review

This pull request introduces a new skill definition for AI agents to assist in setting up FirebaseUI Web across React, shadcn/ui, and Angular projects. The documentation provides detailed workflows, dependency requirements, and implementation patterns for each framework. Feedback from the review focuses on improving the robustness of the provided code snippets by ensuring that all variables, such as firebaseConfig and navigate, and types like ApplicationConfig, are either defined or clearly marked as placeholders to prevent the generation of invalid code.