docs: clarify multiline command continuations

[KirtiRamchandani]

Fixes #1149

Problem

The getting-started guide shows arrays for long argument lists, but it does not show how to split one shell command across multiple lines. This makes it easy to write a backslash followed by trailing spaces and accidentally start a new shell command on the next line.

Root cause

zx passes literal command text to the configured shell. A newline is treated by the shell as a command separator unless it is escaped, and a backslash only continues the line when it is the final character before the newline.

Solution

Add a concise multiline fzf example and call out that trailing spaces after the continuation backslash break the command.

Tests run

• npx prettier --check docs/getting-started.md
• git diff --check HEAD~1..HEAD
• Attempted npm run docs:build; it fails on the current checkout in Vite/esbuild while transforming generated VitePress app destructuring, before this Markdown content is involved.