Add manual XSD downloading steps

[shannynalayna]

Document the steps needed to manually download the GS1 XSD files without Grid's built-in CLI command.

Currently, this will be specific to purchase order XSD files. In the future, may include product.

This includes adding a section to the "Using Purchase Order" feature document to use the grid-download-xsd command.

[ameliabradley]

Spoke with @vaporos and we're just going to add some information to the manpage that says with either it gets its files and puts them (so that users could go through the manual steps if they wish)

IN DEPTH
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[concete explanation of what the command does]