* Add documentation for configuring Packer for HCP Packer
This change works to document the two configuration options for setting
up Packer to publish build artifacts to an active HCP Packer registry.
Currently the HashiCorp Cloud Platform documents the process for
configuring Packer via a HCL template, which is the preferred route.
This documentation focuses more on the use of environment variables for
configuring Packer for HCP Packer, which is necessary for legacy JSON
template users looking to publish to HCP Packer.
In light of the new HCP Packer documentation page we've opted for a shorter
version of the HCP Packer introduction section.
* Applying the first round of suggestions to tighten up the documentation.
* Move HCP Packer side bar to top
* Move intro into docs
* Add redirects for intro pages
Co-authored-by: Laura Pacilio <83350965+laurapacilio@users.noreply.github.com>
