Clarify package addition/installation recommendations

[jkodroff]

Will need to refine this before implementing, but we should be consistent about:

• Any package that needs a local SDK to be generated (component, Any TF provider, etc.) must be added via pulumi package add. This will add it to your Pulumi.yaml.
• Any other package (native language package, published provider SDK, etc.) should be added via the package manager (e.g. npm install @pulumi/aws)
• pulumi install should always be the default method of installing packages b/c it will install both the packages in your package manager's manifest and the packages in your Pulumi.yaml. It will also will work with the local package manager to make sure it "acts right" (e.g. activating a venv).