Some of the layers do not supply a README.org file which caused
invalid links in the layer list. To avoid this I have changed
spacemacs//generate-layers-from-path to only add links to layers
with a valid README.org file.
I have also removed an invalid check to exclude the non existing directory
"distribution" from the layers list. I think that originally the folder
"distributions" should have been excluded but this is not longer
a feasible action as there is at least one layer with a valid README.org
file in there today. So now we add links to all layers providing a
README.org file independent of their category.
