Replace the quickstart with a first completed backtest #109

Affected library: backtest
Current version: 0.1.8
Location: docs/getting-started/quickstart.md and the first tutorial

Problem

The current quickstart uses a linear synthetic trend and buy-and-hold. It reports no completed trades and an implausibly high Sharpe ratio, so a new reader cannot use it to recognize a credible result.

Required outcome

Give a new user a short, executable path from data and a strategy callback to a completed round-trip trade, equity curve, fills, and trades. Explain the execution assumptions and link to the relevant book example.

Acceptance criteria

• The installed-wheel example finishes in under one minute with at least one completed round trip.
• Every displayed metric and result artifact matches execution with the stated package version and input.
• A reader can identify the feed columns, order, fill timing, equity, trade, and fill from the page.
• The strict MkDocs build and documentation example check pass.

Depends on the shared tutorial data and example checks.