Publishing Packages: Add examples to index and Installation & Configuration pages

[AaronFriel]

The how-to-author page's section on Overview, installation & configuration does not define contain examples of _index.md or installation-configuration or tell the user what to do with these files once authored.

• Linking to what we consider canonically good overview and install configuration documentation would resolve this ambiguity.
• _index.md is not part of the provider repository that the user owns today, rather, it's stored in github.com/pulumi/registry. That should be clarified, along with an example pull request or commit showing how to add a provider to the registry.