Remove support for HTML content (index.html)

[caugner]

We are now 100% Markdown, so we no longer need the HTML-to-Markdown conversion.

[caugner]

Never mind, it was already removed in #6849.

[yin1999]

There is still some html files in ru, I'll go to resolve them :)

[yin1999]

There is still some html files in ru, I'll go to resolve them :)

After resolving that, we are 100% markdown now. 🎉

[caugner]

Closing as won't fix, as Rari replaced the Yari build system, and no longer supports index.html.