3.x: [Wiki] Improve Operator-Matrix with links and notes

[akarnokd]

Update the OperatorMatrixGenerator to add anchors and links to notes about why operators are not present in one or another class.

https://github.com/ReactiveX/RxJava/wiki/Operator-Matrix

Some operators are missing and will be added as part of #6852.

Resolves #6855.

[codecov]

Codecov Report

Merging #6857 into 3.x will decrease coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6857      +/-   ##
============================================
- Coverage     98.29%   98.25%   -0.04%     
+ Complexity     6381     6380       -1     
============================================
  Files           715      715              
  Lines         46524    46524              
  Branches       6387     6387              
============================================
- Hits          45731    45713      -18     
- Misses          281      293      +12     
- Partials        512      518       +6

Impacted Files	Coverage Δ	Complexity Δ
...tivex/rxjava3/internal/jdk8/ParallelCollector.java	89.9% <0%> (-3.67%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.1% <0%> (-2.61%)	2% <0%> (ø)
...tivex/rxjava3/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
.../operators/observable/ObservableFlatMapSingle.java	93.65% <0%> (-1.59%)	2% <0%> (ø)
...ava3/internal/operators/maybe/MaybeMergeArray.java	96.04% <0%> (-1.13%)	6% <0%> (ø)
...rxjava3/internal/schedulers/ExecutorScheduler.java	95.89% <0%> (-0.92%)	10% <0%> (ø)
...perators/mixed/ObservableConcatMapCompletable.java	99.18% <0%> (-0.82%)	3% <0%> (ø)
...3/internal/operators/flowable/FlowableGroupBy.java	95.59% <0%> (-0.63%)	3% <0%> (ø)
...a/io/reactivex/rxjava3/subjects/ReplaySubject.java	97.14% <0%> (-0.41%)	50% <0%> (ø)
...ava3/internal/operators/parallel/ParallelJoin.java	91.66% <0%> (-0.38%)	3% <0%> (ø)
... and 9 more

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