doc: Put binaries first and link to Trac wiki on the download page

[atlight]

Fixes osgeo/gdal#9065.

I assume most users will want binaries, not source, so I'm putting the binaries section first.

For information on binaries (other than Conda builds), users are directed to the Trac wiki page, which is being continually updated by those who build GDAL binaries (https://github.com/OSGeo/gdal/issues/9065#issuecomment-1889250617).