Document node 18 dependecies

[TatianaKapos]

Suggestion

We bumped node from 16 to 18 in what will be .73. We need to make sure we update our documentation to include this dependency

Location

https://microsoft.github.io/react-native-windows/docs/rnw-dependencies

[chrisglein]

Current instructions don't specify a version of Node. But it should match what the script does, which chooses an explicit version.