Add alt text for og:image

[bkmgit]

What happened?

• Minimal Mistakes version: 4.24.0
• Ruby gem or remote theme version:
• Jekyll version:
• Git repository URL: https://github.com/mmistakes/minimal-mistakes
• Hosted on GitHub Pages (if yes provide URL to site): https://mmistakes.github.io/minimal-mistakes/
• Operating system:

Expected behavior

An option to specify alt text for og:image would make these accessible to screen readers. See https://ogp.me/#structured

Steps to reproduce the behavior

Information about creating a teaser image and logo is available at https://mmistakes.github.io/minimal-mistakes/docs/configuration/#site-default-teaser-image however https://github.com/mmistakes/minimal-mistakes/blob/master/_includes/seo.html#L70 does not have alt text

Other

This will complicate setting up multilingual sites, but may be done as a separate issue.