add release 6.0 to www index & download pages

Level Zero: fix images

* fix argument order of getImagefillKernel
* fix SHA1 calculation of memfill/imagefill kernels
* pass image array size to level zero
* for fillImage, use the original pixel value not the converted
* fix some debug messages

this makes tests/kernel/test_image_query_funcs pass.

fix docker files

Moved the release notes under docs

Looks nice there, linking to it from the release
announcement.

apply patch from O. Hartmann: fix build on FreeBSD CURRENT

Update PoCL version to 5.0

CMake: rename variable

Update PoCL version to 4.0

Bump sover for 3.1 and drop -pre version suffix for release

3.1 release preparations