Move the "Creating Packages" section of the documentation to the Julia manual

[LilithHafner]

In general I think things like this are better off in the Julia manual. In some sense, Pkg is "just" a package manager and someone could in theory write another better one. And we should IMO then try to keep the documentation to package management and less about general julia package information. I realize that things are pretty muddy now (for example the extension documentation is in Pkg, which it probably shouldn't be), but I think it is something to strive for.

Originally posted by @KristofferC in #3511 (comment)

This page is not Pkg specific: https://pkgdocs.julialang.org/v1/creating-packages/