docs: add gh-devlake CLI to Getting Started as alternative installation method

[ewega]

Summary

Add gh-devlake — a GitHub CLI extension for deploying, configuring, and monitoring Apache DevLake — as a third installation option alongside Docker Compose and Helm.

What is gh-devlake?

gh-devlake is an open-source gh CLI extension that automates the full DevLake lifecycle from the terminal:

• Deploy locally (Docker Compose) or to Azure (Container Instances + MySQL Flexible Server)
• Create authenticated connections to GitHub and GitHub Copilot
• Configure DORA project scopes with sensible defaults
• Trigger and monitor data syncs
• No web UI required for initial configuration

Support for additional plugins (GitLab, Jira, Jenkins, etc.) is coming soon.

Proposed changes

1. README.md (this repo) — Add a third bullet under "Getting Started > Installation" pointing to gh-devlake as a CLI alternative
2. docs/GettingStarted/QuickStart.md (incubator-devlake-website) — Add gh-devlake as a third option in Step 1 with a brief install snippet

Why

Many developers and platform engineers prefer terminal-first workflows. Adding a CLI option alongside the existing Docker Compose and Helm methods lowers the barrier for users who want to get DevLake running quickly without interacting with the Config UI.