add a "docs" field to models (#1671)

[beckjake]

Fixes the dbt-core side of #1671

It's parsed from the "models" block in schema.yml. It contains one field, a bool named "show" which defaults to True.

version: 2

models:
  - name: model
    description: "The test model"
    docs:
      show: false

This has no impact besides adding the field to compiled output, and this PR does not update the docs page to make any use of the field.

[mike-weinberg]

@beckjake @drewbanin just curious if you saw #1671 (comment) -- considerations for forward compatibility with potential future doc configuration features

[beckjake]

@mike-weinberg I didn't notice it, but show vs hide is more of an aesthetic thing, right? And with this model it seems pretty straightforward to do the same thing as you suggested, except inverted (opt-in instead of out once you commit to making changes):

docs:
   show: lineage

or

docs:
    show:
        - lineage

or whatever else we want to support. So I guess I feel pretty good about forwards compatibility, unless there's something I'm missing?

But of course I will defer to drew on this - I don't have any strong feelings on the matter, and it's easy enough to change.

Edit: Obviously, we'd have to change the code to accept a bool or an enum, but this is a "we could support with this syntax", not a "we will support".

[beckjake]

I'll rebase this - I kind of expected removing docrefs to conflict with this new field.

[drewbanin]

@mike-weinberg let me hit you back in the open issue

[drewbanin]

This LGTM - ship it!