Add "WebPage" type to JSON-LD

[tomschr]

Additional entry which creates pointers to all enabled formats. Looks like this:

"about": [
  {
     "@type": "WebPage",
     "url": "https://documentation.suse.com/sles/15-SP5/html/SLES-raspberry-pi/",
     "encodingFormat": "text/html"
  },
  {
     "@type": "WebPage",
     "url": "https://documentation.suse.com/sles/15-SP5/pdf/SLES-raspberry-pi_en.pdf",
     "encodingFormat": "application/pdf"
  },
]

It's extracted from the Docserv config file. The URL is comprised from these Docserv config elements:

• <product productid="..."> => $productid
• <docset setid="..."> => $docset
• <format .../> = $format
• <dc> / <subdeliverable> => $filename
• <language lang="..."> => used in the filename for PDF

https://documentation.suse.com/$productid/$docset/$format/$filename