gh-completion: add page (#5134)

Co-authored-by: bl-ue <54780737+bl-ue@users.noreply.github.com>

pages/common/gh-completion.md

@@ -0,0 +1,20 @@
+# gh completion
+
+> Generate shell completion scripts for GitHub CLI commands.
+> More information: <https://cli.github.com/manual/gh_completion>.
+
+- Display the subcommand help:
+
+`gh completion`
+
+- Print a completion script:
+
+`gh completion --shell {{bash|zsh|fish|powershell}}`
+
+- Append the `gh` completion script to `~/.bashrc`:
+
+`gh completion --shell {{bash}} >> {{~/.bashrc}}`
+
+- Append the `gh` completion script to `~/.zshrc`:
+
+`gh completion --shell {{zsh}} >> {{~/.zshrc}}`