Make the HTML file template more useful

Summary:
This commit removes the empty meta tag and adds the following:
* a DOCTYPE declaration,
* an html[lang] attribute,
* a document charset declaration,
* a newline at the end of the document.

Adding the DOCTYPE and charset make the template HTML5‐complant.

BUG: 419935

Reviewers: #frameworks, dfaure, ognarb

Reviewed By: ognarb

Subscribers: kde-frameworks-devel

Tags: #frameworks

Differential Revision: https://phabricator.kde.org/D29084