docs: add examples for command line flags including --prefix

[MaxBlack-dev]

Description

This PR adds concrete examples for command line flags, specifically documenting the --prefix\ flag that many users were unable to find in the documentation.

Changes

• Added a 'Common examples' section under Command Line Flags
• Documented --prefix\ flag with a clear use case example
• Included other common flag examples (--global, --save-dev)
• Added a note explicitly stating that any config option can be used as a command line flag

Fixes

Closes #8197

Context

Users were unable to find documentation for the --prefix\ flag, which is a commonly used option for running npm commands in different directories. While the general mechanism (any config option can be a flag) was explained, there were no concrete examples making it hard to discover. The --prefix\ flag is particularly useful in monorepos and CI/CD scripts where you want to run npm commands in subdirectories without changing the current working directory.

Type of Change

• Documentation update

[ljharb]

Note that it's a very bad idea to use --prefix, so it would have been better not to document this at all imo.