a single documentation index for all installed paeckages
This is a feature request.

nimble should be able to generate a single index for all installed packages. The idea is that the index looks like [theindex](https://nim-lang.org/docs/theindex.html) but it will be offline and it will include all packages that are installed via nimble.

when there are multiple versions installed of a single package, I think for the documentation just to have the newest version would be enough for the beginning.


Just as a reference the go programming language already does this: https://godoc.org/golang.org/x/tools/cmd/godoc

related issue: https://github.com/nim-lang/Nim/issues/9444

