docs: Write Getting Started documentation (EN)

[100gle]

Parent Issue

Part of #2 — Phase 3: Documentation Content (EN)

Objective

Create the Getting Started section — the first docs a new user reads.

Source Material

• Current README.md: Quick Start section, Configuration table, CLI table
• Current docs/deployment.md: Prerequisites, Runtime Modes
• Current docs/channels/cli.md: CLI commands reference
• env.example: Full environment variable listing
• pyproject.toml: Dependencies and project metadata

Files to Create

1. src/content/docs/en/getting-started/installation.mdx

• Prerequisites (Python 3.12+, uv or pip)
• Install via pip: pip install bub
• Install from source: git clone ... && uv sync
• Verify installation: bub --help
• Optional: Docker setup reference (link to deployment guide)

2. src/content/docs/en/getting-started/quick-start.mdx

• First run: bub chat (interactive REPL)
• One-shot: bub run "hello"
• Gateway mode: bub gateway
• Comma commands: ,help, ,skill, ,fs.read
• What just happened? (brief explanation of the turn pipeline)

3. src/content/docs/en/getting-started/configuration.mdx

• Environment variables table (BUB_MODEL, BUB_API_KEY, BUB_API_BASE, etc.)
• .env file setup
• Model provider configuration (OpenRouter, OpenAI, custom)
• API format options (completion, responses, messages)
• BUB_CLIENT_ARGS escape hatch
• Channel-specific config (Telegram token, allowlists)

Frontmatter Template

---
title: "Installation"
description: "Install Bub and get ready to run your first agent."
sidebar:
  order: 1
---

Acceptance Criteria

• All three files created with correct frontmatter and sidebar ordering
• Content is accurate against current codebase
• Code blocks have correct language tags (bash, python, toml)
• Internal links between the three pages work
• No dead links to external resources

[100gle]

✅ Completed — en/getting-started/index.mdx and en/getting-started/installation.mdx in place with real content migrated from docs/index.md. zh-cn mirrors created.