Bump MLJOpenML compat and add new methods to manual

[ablaom]

https://github.com/JuliaAI/MLJOpenML.jl/releases/tag/v1.1.0

Currently doc generation will fail:

• Waiting for Trouble with categorical(v) when v is a SentinelArrays.ChainedVector JuliaData/CategoricalArrays.jl#361

Now using OpenML.jl instead of MLJOpenML which sidesteps use of CSV and whence this issue.

[codecov-commenter]

Codecov Report

Merging #821 (92c0a9e) into dev (35f44d4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #821   +/-   ##
=======================================
  Coverage   28.84%   28.84%           
=======================================
  Files           2        2           
  Lines          52       52           
=======================================
  Hits           15       15           
  Misses         37       37           

Impacted Files	Coverage Δ
src/MLJ.jl	100.00% <ø> (ø)

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[ablaom]

Now waiting for:

• CSV to tag new release (currently at 0.8.5) to resolve pkg conflict caused by docs dependency on RDatasets.