SI-6503 Fix scaladoc output to generate valid HTML5

I checked the generated HTML with the validators provided by
http://validator.w3.org/ and http://html5.validator.nu/ to verify that
all issues have been fixed.

Changes:
- Adapt doctype
- Remove processing instructions
- Add alt attributes to image tags

Before that, the output tried to be XHTML 1.1, but failed validation as
attested by both validators mentioned above, making it more likely to
trigger the quirks modes in browsers.
Merge pull request #2533 from soc/SI-6503

SI-6503 Fix doctype, remove processing instructions, ...