Packages: Fix location and explain `pulumi install` usage.

[jkodroff]

• The packages concepts page appears under concepts in the nav, but it's under guides in the filesystem. Move it under concepts in the file system.
• The packages concepts page needs instructions that say pulumi install is the right way to install packages (as opposed to npm i, etc.) b/c it will handle all references, whether they are in package.json (or whatever your supported package manager uses to include dependencies) and any local packages in Pulumi.yaml. We could either separate the topics of adding references to packages and installing packages, or put them together - whatever reads best.
• Make sure the guidance is summarized before we go into depth:
  • Add references for any external libraries or Pulumi packages that have published SDKs with the standard package manager methods (e.g. npm i, adding a line to requirements.txt, etc.)
  • Add references to Pulumi local packages (components, Any TF provider, etc.) via pulumi package add.
  • Install packages (e.g. after cloning a repository) with pulumi install.
• Check for other places where we talk about using pulumi package add and ensure that pulumi install is mentioned as the right way to install dependencies downstream.