Add marble diagrams to a few Single.doOnX methods.

[VeskoI]

This should tick a few tasks from this issue: #5788.

The urls are a bit ugly, as they're from the uploads in the issue. I'm happy to update the urls with different ones if there's a better place to upload the diagrams.

[akarnokd]

Thanks for the PR. There are a couple of things to be changed on the diagrams. Once done, I'll upload them into this repo and provide you with the proper URLs so you can update this PR.

[VeskoI]

Sure, will get these fixed tonight :)

[akarnokd]

Images should be 640 width and the height should be reduced to keep the aspect ratio. In this case, 2079x945 -> 640x290

[VeskoI]

Hope these are better:

[codecov]

Codecov Report

Merging #5987 into 2.x will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #5987      +/-   ##
============================================
+ Coverage     98.22%   98.25%   +0.02%     
+ Complexity     6053     6051       -2     
============================================
  Files           656      656              
  Lines         44078    44078              
  Branches       6118     6118              
============================================
+ Hits          43297    43309      +12     
  Misses          235      235              
+ Partials        546      534      -12

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Single.java	100% <ø> (ø)	146 <0> (ø)	⬇️
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-3.23%)	9% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.33% <0%> (-1.94%)	2% <0%> (ø)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...java/io/reactivex/processors/PublishProcessor.java	98.19% <0%> (-1.81%)	42% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...ernal/operators/flowable/FlowableFromIterable.java	94.11% <0%> (-1.07%)	5% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
...vex/internal/operators/flowable/FlowableCache.java	97.94% <0%> (-0.69%)	10% <0%> (-1%)
... and 17 more

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[akarnokd]

Great, here are the links:

https://raw.githubusercontent.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.doFinally.png
https://raw.githubusercontent.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.doOnSuccess.2.png
https://raw.githubusercontent.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.doOnError.2.png
https://raw.githubusercontent.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.doOnDispose.png
https://raw.githubusercontent.com/wiki/ReactiveX/RxJava/images/rx-operators/Single.doOnSubscribe.png

Some images existed but we don't change those to avoid conflicts with the ReactiveX site linking to them.