kubectl-completion: add page

[girdharagrawalbro]

Adds documentation for kubectl completion command.

Examples included:

• Generate bash completion
• Generate zsh completion
• Generate fish completion
• Generate PowerShell completion
• Load bash completion immediately
• Load zsh completion immediately

#17606

[Managor]

Suggested change

	- Load Bash completions in the current shell session:
	`source <(kubectl completion bash)`
	- Generate Zsh completion script:
	`kubectl completion zsh`
	- Load Zsh completions in the current shell session:
	`source <(kubectl completion zsh)`
	- Generate Zsh completion script:
	`kubectl completion zsh`
	- Load Zsh completions in the current shell session:
	`source <(kubectl completion {{bash|zsh}})`

These could be condensed into a single example