3.x: Add Maybe marble diagrams (14 cases)

[akarnokd]

Related #5806

[codecov]

Codecov Report

Merging #6730 into 3.x will decrease coverage by 0.07%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #6730      +/-   ##
============================================
- Coverage     98.18%   98.11%   -0.08%     
  Complexity     6190     6190              
============================================
  Files           677      677              
  Lines         44663    44663              
  Branches       6171     6171              
============================================
- Hits          43852    43820      -32     
- Misses          285      309      +24     
- Partials        526      534       +8

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/rxjava3/core/Maybe.java	100% <ø> (ø)	172 <0> (ø)	⬇️
...a3/internal/operators/flowable/FlowableCreate.java	90.29% <0%> (-7.12%)	6% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	92.7% <0%> (-2.19%)	2% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	95.94% <0%> (-1.36%)	2% <0%> (ø)
...rnal/operators/flowable/FlowableFlatMapSingle.java	95.34% <0%> (-1.17%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	96.25% <0%> (-1.07%)	5% <0%> (ø)
...rxjava3/internal/schedulers/ExecutorScheduler.java	96% <0%> (-1%)	10% <0%> (ø)
...3/internal/operators/flowable/FlowablePublish.java	97.53% <0%> (-0.99%)	16% <0%> (-1%)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-0.86%)	2% <0%> (ø)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.84%)	3% <0%> (ø)
... and 11 more

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