CI: Add build docs

[eksperimental]

No description provided.

[whatyouhide]

Is the idea with this to build docs just to ensure that there are no errors when building docs?

[josevalim]

@whatyouhide yes, because it was broken due to a couple ExDocs commits prior. We can also add --warnings-as-errors in the future to handle invalid docs. :) Does it look good to you?

[josevalim]

💚 💙 💜 💛 ❤️

[eksperimental]

@whatyouhide yes, because it was broken due to a couple ExDocs commits prior. We can also add --warnings-as-errors in the future to handle invalid docs. :) Does it look good to you?

@whatyouhide this is the issue in ExDoc for reference elixir-lang/ex_doc#1539