docs: add shell completion setup to getting started guides #657
Description
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.mddocs/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.