There's a thread here that should help.
Essentially the suggestion is to have a top-level index.html
that links or redirects the reader to the one in the doxygen folder. I note you're new to HTML but it's quite simple.
There's an example of a minimal index.html
you could use towards the bottom of that thread which I'll replicate here in case that thread ever disappears. It was contributed by Clemens Feige.
<head>
<meta http-equiv="refresh" content="1; URL=doxygen/index.html">
</head>
<body>
<p>You will be redirected automatically, otherwise please <a
href="doxygen/index.html">click here</a>.</p>
</body>
You'll need to tweak the paths according to your set-up.