Hugo modules - move overview content to an introduction page

[jmooring]

https://gohugo.io/hugo-modules/

See also #2087

[chrillek]

Currently, the introduction to modules reads

Hugo Modules are the core building blocks in Hugo. A module can be your main project or a smaller module providing one or more of the 7 component types defined in Hugo: static, content, layouts, data, assets, i18n, and archetypes.
You can combine modules in any combination you like, and even mount directories from non-Hugo projects, forming a big, virtual union file system.
Hugo Modules are powered by Go Modules. For more information about Go Modules, see:
https://github.com/golang/go/wiki/Modules
https://go.dev/blog/using-go-modules

I think that the first sentence should be immediately followed by an explanation why one would want to use modules. Why would I want to have a "big, virtual union file system" instead of a "small, non-virtual directory" for my project? Referring people to the Go documentation is, in my mind, not very helpful. I find this documentation difficult to read, lacking examples and adding an additional layer of complexity: I don't want to learn about Go, I want to use Hugo.

Also, "you can combine modules in any combination" might be better written like "you can combine modules in any way" – anything that avoids combining combine and combination, because one doesn't combine combinations – combining something results in a combination.