docs: convert bare next CLI commands to package manager snippets
Convert all bare `next <command>` code blocks in the CLI reference page
to use the `package="..."" syntax that renders tabbed package manager
variants (pnpm/npm/yarn/bun). Previously these commands wouldn't work
if copy-pasted directly into a terminal since `next` is not a global
command.
Co-authored-by: Joseph <icyJoseph@users.noreply.github.com>