Refactor blocks/cover and pageinfo shortcodes

[chalin]

• Fixes Shortcodes: drop support for pre-Hugo-0.54.x behavior of {{% %}} #939 for blocks/cover and pageinfo
• Closes shortcode 'blocks/cover': improvements #1400 by superseding it. This PR offers a simpler solution.
• blocks/cover: drops the test for the page's file extension, and drops the use of markdownify. Let's the Markdown processor handle markdown content
• Preview:
  • https://deploy-preview-2480--docsydocs.netlify.app/about/
  • https://deploy-preview-2480--docsydocs.netlify.app/docs/content/shortcodes/#pageinfo

Only non-whitespace change to the generated site files is due to the updated text in the UG's shortcodes page:

$ (cd public && git diff -bw --ignore-blank-lines -I 'buildDate' -- ':(exclude)*.xml' ':(exclude)**_print*' ':(exclude)**xx*') | grep ^diff 
diff --git a/docs/content/shortcodes/index.html b/docs/content/shortcodes/index.html