My understanding is that you are trying to execute the above command in the SHbundle folder.
The problem is that Pkg.instantiate() is not going to work since it does not know how to find Xnumber.
Before instantiating you should Pkg.develop(url="https://gitlab.com/vyush/Xnumber.jl") first.
Try this:
using Pkg
Pkg.Registry.update()
Pkg.develop(url="https://gitlab.com/vyush/Xnumber.jl")
Pkg.develop(PackageSpec(path=pwd())) # I'm assuming we are in the SHbundle directory
Pkg.instantiate()
using Documenter: doctest
using SHbundle
doctest(SHbundle)
include("docs/make.jl")