Reintroduce improved vitamin D recommendation

[jaysonvirissimo]

A similar section was introduced here, but then later removed. This comment indicates the removal was intentional, but doesn't itself provide a rationale. This set of changes (1) brings back the old section, (2) reorganizes it, and (3) supplies additional warrant for the recommendation in the form of a recently released RCT pertaining to COVID-19 specifically (rather than the mere analogical reasoning from respiratory diseases in general to the coronavirus in particular in the original version).

[jaysonvirissimo]

I expect @emersonthis and @jmcmurry to have the most context about whether this section should be included/not.

[emersonthis]

At the time when we removed it, it wasn’t clear that it met the criteria for advice to be included in the site. We were trying to focus on COViD-specific recommendations. Sounds like maybe it’s more relevant than we originally thought. There’s a doc in the linked issue that talks more about that decision process.

On Sep 4, 2020, at 9:59 PM, Jayson Virissimo notifications@github.com wrote:

 I expect @emersonthis and @jmcmurry to have the most context about whether this section should be included/not.

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[jaysonvirissimo]

I see. In that case, I think it should be re-included, since it now meets all four criteria mentioned in the linked issue.

[jaysonvirissimo]

@nditada, @rousik: CI's attempt to deploy a staging build via vercel is failing for lack of creds (Error! No existing credentials found.). Is that something you can re-run for me?

[jmcmurry]

I didn't realize that this didn't make it in and agree it should be there. I can force a merge