(doc) Update puppet.installDirectory usage
(doc) Update puppet.installDirectory usage

This commit updates references to `puppet.puppetAgentDir` to
`puppet.installDirectory` to remove references to legacy settings. It
also updates the configuration section showing the different install
directories that are stored for different platforms to not show a red
error in markdown.

jpogran/puppet-vscode,glennsarti/puppet-vscode,jpogran/puppet-vscode