docs: rewrite getting started pages for a better first 5 minutes

[bartlomieju]

Summary

Rewrites all five "Getting Started" pages to be more code-first and action-oriented, inspired by feedback comparing Deno's onboarding experience to Bun's.

• Welcome page: leads with a runnable HTTP server example, shows all CLI tools upfront, adds "No package.json, no build step, no node_modules" hook
• First project: replaces add(2, 3) with a progressive tutorial: init → HTTP server → routing → add zod from npm → write and run tests
• Installation: streamlines to 1-2 recommended methods per OS, collapses niche options (MacPorts, asdf, vfox, cargo), adds inline Docker usage with a Dockerfile example
• CLI overview: trims to essentials (run, permissions, arguments, watch mode), replaces plain text table with a linked subcommands table
• Environment setup: leads with VS Code + JetBrains, collapses all other editors into expandable sections, adds --dynamic completions flag for deno task

Net result: -689 lines, +448 lines across 5 files.

Test plan

• Verify all pages render correctly locally (deno task dev)
• Check internal cross-links resolve (CLI reference pages, fundamentals pages)
• Walk through the first project tutorial end-to-end
• Confirm collapsible <details> sections work for editors and install methods
• Verify the Docker table and Dockerfile example render properly

🤖 Generated with Claude Code