Don't use frames

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.

Search the generated HTML pages for frame usage and read how those pages are produced. Confirm the replacement preserves access to the reference-page index and content without frames, then verify representative pages in a browser.

Written by the indexing model from the issue text.