Remove empty space at the top of OpenGL pages

[TestingPlant]

This moves the bottom.php file to the index.php file, which seems to remove the empty space.

Fixes #92

[oddhack]

Sorry, overlooked this for a long time. Could you check if this is still happening? I just accepted an MR which fixes some PHP include paths, and the top subwindow now contains the navigation interface, as it should - I'm not sure if this is the white space you refer to though. Thanks.

[eloj]

I can't really try this PR, but there's definitely a big white block on top in Firefox 147 when I open pages at https://registry.khronos.org/OpenGL-Refpages/gl4/

Shown here in the 'take screenshot' mode to make it more visible.

I can tell in inspect mode that it's the top.php frame, but the HTML document consists of a head containing only title ("OpenGL SDK"), and then empty body tags. Which makes perfect sense if it's including gl4/top.php since that is exactly what that document is.