2.x: Update Getting started docs

[ashishkrishnan]

This patch the following changes to the Getting started docs as per Issue No: #6132

• Remove Troubleshooting guide
• Bump RxJava versions to 2.2.0 for Gradle, Maven etc.

Questions:

• Looks like JFrog for RxJava 2 is broken or invalid now. Can we add Jitpack?
• Need suggestions for Getting started from readme.md. Although, How to use RxJava or Alphabetical List of Operators, Additional-Reading.md can be great next steps.

[akarnokd]

JFrog: The main readme contains the proper links: https://github.com/ReactiveX/RxJava#binaries

[codecov]

Codecov Report

Merging #6136 into 2.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6136      +/-   ##
============================================
+ Coverage     98.25%   98.26%   +0.01%     
+ Complexity     6197     6195       -2     
============================================
  Files           667      667              
  Lines         44853    44853              
  Branches       6213     6213              
============================================
+ Hits          44069    44075       +6     
+ Misses          237      235       -2     
+ Partials        547      543       -4

Impacted Files	Coverage Δ	Complexity Δ
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
.../io/reactivex/disposables/CompositeDisposable.java	98.14% <0%> (-1.86%)	39% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-1.64%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	98.61% <0%> (-1.39%)	57% <0%> (-1%)
...ternal/operators/observable/ObservableFlatMap.java	86.9% <0%> (-0.96%)	3% <0%> (ø)
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <0%> (-0.85%)	3% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	96.66% <0%> (-0.84%)	2% <0%> (ø)
.../operators/maybe/MaybeFlatMapIterableFlowable.java	97.54% <0%> (-0.82%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableTimeoutTimed.java	98.37% <0%> (-0.82%)	3% <0%> (ø)
...x/internal/operators/flowable/FlowableFlatMap.java	89.47% <0%> (-0.27%)	4% <0%> (ø)
... and 15 more

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[akarnokd]

Getting started from readme.md

Just copy in the from this inclusive to this exclusive.

How to use RxJava

Perhaps change the non-java examples into Java after all. However, since the text should also be reviewed for 2.x terminology, I don't recommend taking this one.

Alphabetical List of Operators

There links there should point to the various category-wiki pages, such as https://github.com/ReactiveX/RxJava/blob/2.x/docs/Creating-Observables.md#just most of such pages hasn't been updated yet. I'd leave this last.

Additional-Reading.md

Beyond verifying the existing links, one should vet the various blog posts. Some of the existing links should be also checked for how much they are version agnostic. I'd leave this to somebody with a longer history with RxJava.

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Perhaps the easiest next step is to work on one of the operator-category listings such as https://github.com/ReactiveX/RxJava/blob/2.x/docs/Filtering-Observables.md - review, remove nonexistent operators, fancy like #6131

[ashishkrishnan]

@akarnokd Thanks for the next steps. Will pick them. Anything more to be done on this PR?

[akarnokd]

We can always expand it later, the important changes are already okay with it.

[ashishkrishnan]

Yes. That's true. Looking forward to it.