Issue title: Don't use frames
 Issue body: Hi. 

It would be good to remove the use of frames from the pages. 

Reasons:
1. I've been using this reference for years (thank you!), but only today did I come via the OpenGL registry (and thus loaded the page via the frames), the usual way is by finding the page I want using google for [example](https://www.khronos.org/registry/OpenGL-Refpages/gl4/html/glBindVertexBuffer.xhtml). So, this is the first time ever I've seen the index to the left of the page content... I've been quietly annoyed at this lack of an index, but now I understand how this came to be.
 
2. Also, frames are deprecated in the web standard, to quote [Mozzila](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/frame):
> This feature has been removed from the Web standards. Though some browsers may still support it, it is in the process of being dropped. Avoid using it and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.


 Comments: 
 Comment 0: I can give fixing it a go. I don't have any time either, but I had a quick look and happily you seem to be usign php which I have some familiarity with, and it looks fairly simple. Afaik, the answer is not iframes anyway as they are more intended for including external content. My thinking is to just pipe everything through the index.php and send the page to display along as an argument and include the index and the other parts using normal php includes or whatever they are called. If we were to move the current xhtml we can possibly fix the links everyone has (and google has in its index) so they redirects to the main page. That'd be a nice bonus. Not going to promise any ETA :)
 Comment 1: Thanks. Eventually I hope to migrate the ref pages to asciidoctor and HTML5 output; for now, we're stuck with the XHTML outputs.
{'source': 'GithubIssues', 'repo_name_with_owner': 'KhronosGroup/OpenGL-Refpages', 'issue_id': 308753805, 'repo_stars': 400.0}