docs: rewrite first-run page

[YouyoungSeo]

Summary

• Rewrite Step 1 (Create a Project) to describe the actual "No project open" screen and its three entry points, with screenshot
• Rewrite Step 2 (ui_information.md) with clearer bullet breakdown and links to Agents & prompts / Prompting best practices, with screenshot
• Rewrite Step 3 (Open the example tests) to list all template files (login test, cross-platform test, test data, rules/setup/teardown), with screenshot
• Rewrite Step 4 (Run it) to reference the actual Run · Local button, with screenshot
• Remove remaining em-dashes throughout the page

Test plan

• Verified locally with pnpm dev at /docs/get-started/first-run