Add og:image:alt for accessibility.

[aartaka]

Add image alt text in index.html

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

This ensures that the image added via OpenGraph <meta> tag has a proper alt text for use with screen readers or image-less user agents. Leaving the alt text empty is a sane fallback for when the image is purely decorative too.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Documentation and tests are irrelevant for such a change, right?

[roblarsen]

I'll have to take a look at this! Thanks for your submission.

[coliff]

The HTML file needs to be updated in the src/ folder too.
Other than that, 👍 from me.
REF: https://ogp.me/#structured

[aartaka]

Added the code to src/index, docs/html and docs/extend, seems that I've covered everything.

I barely resisted the temptation to add Twitter cards image alt too, but that's for a different PR!

[roblarsen]

Thanks!