Defer Hero Images for Better Loading Speed

[loreanvictor]

Hero images are not essential to the content, and they take long to load on low internet speeds, blocking access to the whole page. They should be deferred so that they don't block the page and main content.

[loreanvictor]

this commit will speed up loading of hero images as it now uses <picture> tag to avoid loading different sizes for responsive heroes. caveats:

• <picture> is not supported on IE 11 (so no more hero images on IE 11)
• the same approach is not applicable to dark-mode light-mode hero images since the OS might not support dark mode / light mode settings and so media queries cannot be applied to read such settings.