3.x: Add operator matrix wiki + auto-extractor

[akarnokd]

Adds the Operator-Matrix.md wiki page and some code to generate the matrix of operators available in various base classes.

It should also help see missing or inconsistent operator presence. Note that many operators do not make sense everywhere.

[codecov]

Codecov Report

Merging #6845 into 3.x will increase coverage by <.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6845      +/-   ##
============================================
+ Coverage     98.24%   98.25%   +<.01%     
+ Complexity     6381     6380       -1     
============================================
  Files           715      715              
  Lines         46529    46529              
  Branches       6388     6388              
============================================
+ Hits          45714    45715       +1     
- Misses          292      293       +1     
+ Partials        523      521       -2

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	83.8% <0%> (-5.64%)	2% <0%> (ø)
...a3/internal/operators/flowable/FlowableCreate.java	94.82% <0%> (-2.59%)	6% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.02% <0%> (-2.33%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-1.06%)	3% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	98.02% <0%> (-0.99%)	49% <0%> (-1%)
...perators/observable/ObservableMergeWithSingle.java	99.05% <0%> (-0.95%)	2% <0%> (ø)
...ternal/operators/observable/ObservableGroupBy.java	95.08% <0%> (-0.55%)	2% <0%> (ø)
...activex/rxjava3/processors/MulticastProcessor.java	98.25% <0%> (-0.44%)	82% <0%> (-2%)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.28%)	3% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	99.14% <0%> (ø)	2% <0%> (ø)	⬇️
... and 13 more

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[vanniktech]

Nice