Add social media sharing preview

[jmglov]

At the moment, sharing a quickblog URL on Facebook, Twitter, LinkedIn, etc results in just the URL. It would be nice if the link was rendered as a "sharing card" with a title, post summary, and preview image. The feature could work like this:

Social sharing

Social media sites such as Facebook, Twitter, LinkedIn, etc. display neat little preview cards when you share a link. These cards are populated from certain <meta> tags (as described in "How to add a social media share card to any website", by Michelle Mannering) and typically contain a title, description / summary, and preview image.

By default, quickblog adds tags for the page title for all pages and descriptions for the following pages:

• Index: {{blog-description}}
• Archive: Archive - {{blog-description}}
• Tags: Tags - {{blog-description}}
• Tag pages: Posts tagged "{{tag}}" - {{blog-description}}

If you specify a :blog-image URL option, a preview image will be added to the index, archive, tags, and tag pages. The URL should point to an image that is 1200x630 and maximum 5MB in size. It may either be an absolute URL or a URL relative to :blog-root.

For post pages, meta tags will be populated from Description and Image metadata in the document. For example, a post could look like this:

Title: Sharing is caring
Date: 2022-08-16
Tags: demo
Description: quickblog now creates nifty social media sharing cards / previews. Read all about how this works and how you can maximise engagement with your posts!
Image: assets/2022-08-16-sharing-preview.png

You may have already heard the good news: quickblog is more social than ever!
...

The value of the Image field is either an absolute URL or a URL relative to :blog-root. As noted above, images should be 1200x630 and maximum 5MB in size for best results.

Resources for understanding and testing social sharing:

• Meta Tags debugger
• Facebook Sharing Debugger
• LinkedIn Post Inspector
• Twitter Card Validator

[jmglov]

@borkdude I have a PR ready for this as soon as the testing one is merged. :)

[borkdude]

Sounds good to me.