Ported shipping guide documentation from Spree

[Reddshift]

This could be polished a bit more - but Clarke just announced in SolidusConf that he would be hiring a technical writer to work on implementation guides and documentation. so I thought I would create a pull request with what I have so far. If this gets merged and helps some people in the meantime, great! If we all feel like it would be better to wait for a more polished version - great too!

I Started with a copy of the Spree doc was pulled (https://github.com/spree/spree-guides/blob/master/content/developer/core/shipments.md) and made the following edits (among others)

• Confirmed as much as possible that Solidus still works as described in the spree documentation. Fixed as needed.
• Refined descriptions and explanations in (hopefully) appropriate ways
• Only used ticks for things that could actually be in code or typed at a terminal or console.
• Reorganized examples to come after definitions
• Changed the “Note” callout pattern to a bold word with italic text instead of horizontal rules
• Replaced “Spree” with “Solidus” as appropriate

Questions for feedback:

• Stock splitting documentation - I just confirmed the paths are correct and files exist in solidus, I didn’t confirm that function in solidus has not changed
• Should we include the documentation (essentially a recommendation or promotion) of the solidus_active_shipping extension? The extension itself acknowledges that it is old, complex, and needs some work. I didn’t have time to test the example code in solidus, but I did confirm that all objects/classes referred to exist, and the examples look sound.
• Wondering how to appropriately acknowledge that the document originated from the spree guide…

[mamhoff]

Clarkes announcement does not preclude the community from porting Spree documentation. The person that will be hired will have lots to do either way, so I suggest going on with community-delivered guides.

@Reddshift this is an amazing effort, an I would very much like to see this merged as it still is a pretty good guide.

[mamhoff]

Thank you!

[Reddshift]

Sounds good @mamhoff -- porting the Spree guides is fairly low effort compared to the extensive work that could be done to take documentation further. I agree that it is worth doing, even just to have some documentation until someone is hired. I'll try to fit in porting some more...