Document content adapters

[jmooring]

TODO

• [x] Add a "Multilingual" section to the bottom of the page showing the content structure: one example when using one content directory, another example when using multiple content directories
• [x] Revisit "content.type" (now content.mediaType)
• [x] Revisit "markup" front matter field (identifiers)
• [x] Revisit content format classification table (https://gohugo.io/content-management/formats/#classification)
• [x] Describe difference between resource (use existing resource) and string (create new resource) in map passed to AddResource