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