page templates: fix CSS loading, DOCTYPE wrong uppercase HTML, not html

Seems Webengine is more sensitive here and as result ignored to load CSS styles from head links.

I will not tell how much time I spent playing with the actual HTML, until I learnt how to debug via remote web inspector and wondered why no styles were used on the elements at all and from there got suddenly quickly traced the issue down #)

Now the index & glossary pages have the same (header) styling as the application manual pages.

24.02 backport candidate.

@ltoscano