Add thumbnail for Web Forms Preview page

[alyblenkin]

The default ODK image shows up in the thumbnail when I share a link to the preview page.

We want people to see a screenshot of the Web Forms Preview instead.
https://getodk.org/web-forms-preview/

[alyblenkin]

@yanokwa do we still need this?

[yanokwa]

I think it'd be nice to have for when we share the link directly. It's not something I can easily do though, so @eyelidlessness or @brontolosone will have to take this on.

[lognaturel]

@yanokwa This seems like it belongs in the website HEAD, not here?

[yanokwa]

Ugh, you are right. Fixed at https://github.com/getodk/website/pull/654