x/website: define the term `root module` in ref/mod

[perillo]

The documentation for workspaces introduced a new term root module in the Go Module Reference.
The term is used in the glossary for workspace, but it is never defined.

[perillo]

The article https://go.dev/blog/get-familiar-with-workspaces uses the term root module in the first paragraph, but later use the term main module in

• use: adds a module on disk to the set of main modules in a workspace [...]

This is a bit confusing.

[mvdan]

I think the docs mean to use "main module" consistently; there might have been a mistake due to the similar concept that is "root package".

[cherrymui]

cc @matloob @bcmills

[bcmills]

Yes, it would be correct to use “main module” instead, since https://go.dev/ref/mod#minimal-version-selection also uses the term “main modules”:

MVS starts at the main modules (special vertices in the graph that have no version) and traverses the graph

[gopherbot]

Change https://go.dev/cl/421895 mentions this issue: _content: consistently use main module instead of root module