2.x: More Completable marbles (+18), add Completable.fromMaybe

[akarnokd]

This PR adds marble diagrams (#5789) to the following operators as well as adds the missing Completable.fromMaybe operator (fromX of other types were already there, uses existing Maybe.ignoreElement's underlying implementation).

fromAction

fromCallable

fromFuture

fromMaybe

fromObservable

fromPublisher

fromSingle

mergeArray & mergeArrayDelayError

merge

(both Iterable and Publisher variants)

mergeDelayError

(both Iterable and Publisher variants)

never

timer

[vanniktech]

Big thanks for creating all of those marbles.

[codecov]

Codecov Report

Merging #6085 into 2.x will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6085      +/-   ##
============================================
+ Coverage     98.25%   98.29%   +0.03%     
+ Complexity     6194     6193       -1     
============================================
  Files           667      667              
  Lines         44856    44858       +2     
  Branches       6211     6211              
============================================
+ Hits          44073    44091      +18     
+ Misses          242      229      -13     
+ Partials        541      538       -3

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <100%> (ø)	117 <1> (+1)	⬆️
...ava/io/reactivex/processors/BehaviorProcessor.java	95.96% <0%> (-3.14%)	59% <0%> (-1%)
...tivex/internal/schedulers/TrampolineScheduler.java	96.1% <0%> (-2.6%)	6% <0%> (ø)
...nternal/operators/observable/ObservableWindow.java	98% <0%> (-2%)	3% <0%> (ø)
...activex/internal/schedulers/ScheduledRunnable.java	98.07% <0%> (-1.93%)	29% <0%> (-1%)
...ternal/operators/completable/CompletableMerge.java	96.42% <0%> (-1.2%)	2% <0%> (ø)
...io/reactivex/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-1.07%)	5% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	93.61% <0%> (-1.07%)	3% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	98.37% <0%> (-0.82%)	3% <0%> (ø)
... and 20 more

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