EXT links break if trailing slash is in URL

All of the links for extensions are broken if a trailing slash is in the URL.

Links work:
```
https://registry.khronos.org/OpenGL/index_gl.php
```

Links broken:
```
https://registry.khronos.org/OpenGL/index_gl.php/
```

For example, with a trailing slash, for ARB and KHR Extensions by number for \#2 you get this as the link:
```
https://registry.khronos.org/OpenGL/index_gl.php/extensions/ARB/GLX_ARB_get_proc_address.txt
```

Which leads to the same page. Normally an end user will not end up on a page with a trailing slash, but search engines are, and this is creating a large number of bad links:

And yes, it's been an issue for awhile, was just alerted to it.

Can the links be changed to absolute instead of relative?

Reproduce the issue using the working and trailing-slash OpenGL registry URLs shown in the report, then trace how the extension links are generated. Done means extension links remain valid when the registry page is opened with a trailing slash, including the ARB and KHR example.

Written by the indexing model from the issue text.