head: improve the metadata exposed

* Improves the metadata exposed, such as site or post description, site
  or post image. Make use of <https://matuzo.at/blog/html-boilerplate

>.
* Removes extra empty lines or identation due to badly written layout.
  However, minifying at build could be a good idea.
* Improves very slightly the webmanifest file.

Co-Authored-by: iGor milhit <igor@milhit.ch>