Using the library improvements

[josh-padnick]

I met with a real-life customer who used our using the library page. He made a lot of progress with it, but hit a few friction points in particular:

• The guide doesn't mention using a git repo or what kind of general-purpose folder structure to use (e.g. hierarchical by account > region > environment), so the customer wound up figuring out their own. It may be worth either being more opinionated here, creating a separate guide where we talk about our recommended folder structure for both Terraform and Terragrunt, or just mentioning that the folder structure doesn't matter here and anything will do for now.
• The customer needs to decide between Terraform and Terragrunt, but we don't offer any guidance for that. Same options as above. Perhaps it's worth a dedicated page on Terraform vs. Terragrunt under "Working with the library" where we explain that the Library works with either and our recommendations?
• The customer's corporate network blocked Terraform module downloads via SSH because outbound requests to port 22 were blocked. The workaround to this is to use the HTTPS-style module downloads. See this Slack discussion for details, but that doesn't work with module cross-references, so their real solutions was to self-host the library. The guide should explicitly call this out as a warning so if users hit this they know what to do. Ideally, we could include some sample common error messages here to help users recognize when port 22 is blocked (see https://gruntwork.zendesk.com/agent/tickets/110260 and https://gruntwork.zendesk.com/agent/tickets/110264 for examples).