Add more Howtos/tutorials

[aldesantis]

Once we are done with the "basic" guides, we should probably work on creating a cookbook with different tutorials on how to implement certain features or customizations with Solidus. The recipes can be anything: we could advise people to roll their own implementation, use an extension or even provide both options when possible.

Here's a few ideas I collected:

• GDPR/CCPA compliance
• BI tool integration (Tableau, Looker, Blazer)
• Replatform from X
• Content management
• Marketplace/multitenancy
• Global commerce (going international)
• Sending emails with MailChimp, Klaviyo etc.
• Using Solidus with the Citadel architecture
• Using caching properly