docs: add shell completion setup to getting started guides

[luojiyin1987]

Summary

The getting started guides do not currently explain how to enable shell tab completion for opencli.

Problem

Users can discover opencli completion <shell>, but the onboarding docs do not show:

• how to enable completion in Zsh, Bash, and Fish
• that completion works for both built-in commands and site/subcommand names
• the correct Fish startup config usage

Suggested fix

Add a short tab-completion section to both:

• docs/guide/getting-started.md
• docs/zh/guide/getting-started.md

The examples should use the persistent startup-config commands that match the current implementation in src/completion.ts.

Why this matters

This is a low-risk docs improvement that makes first-run UX better and surfaces an existing feature earlier.