Docs for L1+std

[tlienart]

Following discussion on Slack

• in the docs (maybe in the quick start), add a brief example that does lasso with fit!(machine(@pipeline(Standardizer(), MLJLinearModels.LogisticClassifier(penalty = :l1)), X, y) as described in issue L1 and standarization #106.
• In the warning that is printed, direct the user to the above docs.

[ablaom]

Since this is documentation related, let me mention: JuliaAI/MLJ.jl#913