fix documentation + tutorial

[paraynaud]

@dpo the documentation failed because the module lacks a DOCUMENTER_key.

Here are the lines to generate a new one (I added the line for ExpressionTreeForge):

using DocumenterTools

DocumenterTools.genkeys(user="dpo", repo="PartitionedVectors.jl")
DocumenterTools.genkeys(user="dpo", repo="ExpressionTreeForge.jl")

[dpo]

Done.

[paraynaud]

New push to trigger the test + documentation build. I had to make a new push since the last commit was older than 1 month