Document packages.config schema

## Is Your Feature Request Related To A Problem? Please describe.

Once https://github.com/chocolatey/choco/issues/886 is completed, there will be many elements available for the packages.config file. It would be good to have documentation for them. The current parameters are documented in the help output for the choco install command.

## Describe The Solution. Why is it needed?

As suggested in [this comment](https://github.com/chocolatey/choco/issues/2600#issuecomment-1047990635), it would be best to streamline the amount of information in the help command, and instead link to information for larger topics.

Therefore, the information about packages.config should be moved to a separate page on the docs repository.

## Additional Context.

There is a pre-existing shortlink to the packages.config documentation that is a part of the install command help (https://ch0.co/packages_config) that should be updated once the page is live.

## Related Issues

Documentation for chocolatey/choco#886
Counterpart to chocolatey/choco#2600