Document using installType and installDirectory

[jpogran]

The extension works when there is both pdk and agent present, and when only one is present. The problem here is that the settings for the extension are not helping the user choose successful configurations and allowing mixing of settings that then produce incorrect paths to the parts of puppet we need.

The intention for auto was to not use installDirectory or installType, so that the extension would choose whether to use pdk or agent and which paths to use itself. This was intended to make the default 'happy path' the situation where a user has pdk installed to the default path and not require the user to set any settings.

Items to document:

• If you installed either pdk or agent to the default install paths, and you want the extension to figure out whether to use pdk or agent, then do not set any settings.
• If you installed either pdk or agent to the default install paths, but want to choose whether to use pdk or agent, set installType and omit installDirectory.
• If you installed either pdk or agent to custom install paths, you must specify installType as either agent or pdk, and set installDirectory to the correct path.

Originally posted by @jpogran in puppetlabs/puppet-vscode#668 (comment)