Structure "see also" content

[ddbeck]

(there's also talk of structuring "See also", since it's basically just a list of links - and that would enable us to render it in other contexts etc. I guess we should talk about that in yet another issue...)

Originally posted by @wbamberg in https://github.com/mdn/stumptown-content/issues/55#issuecomment-501801114

We ought to have some structure around "see also" content, such that:

• data in stumptown JSON is an array of cross references that clearly identifies other stumptown content (e.g., html.elements.sometag) or a URL (e.g., https://developer.mozilla.org/docs/Web/HTML/Element/sometag) or some combination of those
• authoring is reasonable and unsurprising. This might be a YAML array in meta.yml or a prescribed prose Markdown section (e.g., a single-level unordered list) paired suitable linting

I suspect the next step is to survey existing see-alsos to see how much flexibility we really need there.