doc/External-Commands: update links
doc/External-Commands: update links

`brew-gem`'s link is 404.
`brew-pip` has reached EOL.
Thus I updates the link of `brew-gem`, and remove descriptions
about `brew-pip`.

Thanks @DomT4 for pointing out brew-gem's new home.

Closes Homebrew/homebrew#34569.

Signed-off-by: Jack Nagel <43386ce32af96f5c56f2a88e458cb94cebee3751@gmail.com>

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