Add og-image tags for social media previews

[cmajel]

This is to add og-image tags to the site for social media previews.

Default image in Figma at 1200x630.

[Scotchester]

@cmajel We've deployed the default image! Did you want to leave this open for individual page images?

Note: You won't see it yet if you're trying to link to https://staging--cal-itp-mobility-marketplace.netlify.app/ because of that not using the production domain.

[cmajel]

@Scotchester yes if it's possible to use indiv page images when available and this as a default, that works! If not, just this image should be fine.

[Scotchester]

It's certainly possible! Reopening to track that goal.