SI-6503 Make scaladoc create valid HTML

Until now, scaladoc tried to emit XHTML 1.1,
but failed validation in numerous ways.

The best option seemed to be to move to HTML5,
both in terms of changes necessary and the chance
to be rendered in standards mode in all browsers.

Changes to make this happen:

- Change the doctype to the one for HTML5
- Remove XML processing instructions (not needed anymore)
- Replace old encoding scheme with meta charset attribute
- Add alt attributes to image tags