Move pkg/pillar/docs into top docs directory.

[eriknordmark]

It is hard to find the docs down in pkg/pillar if you don't know they are there.

[rvs]

Agreed @eriknordmark -- I think all the new docs for sure should go under top level docs. There may be things like design docs, etc. that only make sense at the pillar level -- but we don't have those yet, I think.

[rene]

I think this is not an issue anymore. Pillar, as all other packages under pkg/, has its own documentation inside his own folder (which makes sense). Although it could be not easy to find such documentation in the past, currently we have mkdocs in place, which considers all documentation files throughout the whole repository. For example, pillar documentation can be easily accessed through mkdocs (when they are generated), or even more quick by accessing the online version at https://eve-os.readthedocs.io/en/latest/pkg/pillar/

My vote is to close this issue.

FYI: @eriknordmark , @rouming