Add big /docs dir back to export-ignore in gitattributes

[bram-pkg]

The directory is quite big, no need to package that along with the SDK when installing using composer

[ezimuel]

@bram-pkg unfortunately, I cannot remove this folder in .gitatttibutes since the docs folder is used by the process that generates the documentation we pages. We are working to change this and in the future this will be possible. Sorry for the inconvenience.