Update documentation to reflect recent BetaML reorganisation

[ablaom]

#1090 #1089 No new release.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (51ed76e) 57.89% compared to head (24d5bf2) 57.89%.
Report is 23 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1091   +/-   ##
=======================================
  Coverage   57.89%   57.89%           
=======================================
  Files           2        2           
  Lines          38       38           
=======================================
  Hits           22       22           
  Misses         16       16           

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[ablaom]

Failure of integration tests needs:

• Problem with MLJ interface for KMedoidsClusterer sylvaticus/BetaML.jl#67

[ablaom]

to fix doc failure we need:

• add missing model descriptor for PerceptronClassifier (BetaML): self-assigned

ERROR: LoadError: The following keys are missing from /docs/ModelDescriptors.toml: ["PerceptronClassifier_BetaML"]. 
[18](https://github.com/alan-turing-institute/MLJ.jl/actions/runs/7687809013/job/20948276721?pr=1091#step:6:19)
Stacktrace:
[19](https://github.com/alan-turing-institute/MLJ.jl/actions/runs/7687809013/job/20948276721?pr=1091#step:6:20)
 [1] error(s::String)
[20](https://github.com/alan-turing-institute/MLJ.jl/actions/runs/7687809013/job/20948276721?pr=1091#step:6:21)
   @ Base ./error.jl:35
[21](https://github.com/alan-turing-institute/MLJ.jl/actions/runs/7687809013/job/20948276721?pr=1091#step:6:22)
 [2] top-level scope
[22](https://github.com/alan-turing-institute/MLJ.jl/actions/runs/7687809013/job/20948276721?pr=1091#step:6:23)
   @ ~/work/MLJ.jl/MLJ.jl/docs/make.jl:33
[23](https://github.com/alan-turing-institute/MLJ.jl/actions/runs/7687809013/job/20948276721?pr=1091#step:6:24)
in expression starting at /home/runner/work/MLJ.jl/MLJ.jl/docs/make.jl:33
[24](https://github.com/alan-turing-institute/MLJ.jl/actions/runs/7687809013/job/20948276721?pr=1091#step:6:25)
ERROR: LoadError: failed process: Process(`julia --project=docs docs/make.jl`, ProcessExited(1)) [1]

cc @sylvaticus