2.x: Expand Creating-Observables.md wiki

[akarnokd]

This PR expands and updates the Creating-Observables.md wiki page to 2.x terminology and feature set.

This also serves as a discussion basis about how to detail the various operators. The PR suggests the following structure:

• Top outline with links to the family of operators
• Each operator lists:
  • an availability list for the various reactive types: available, not available
  • ReactiveX documentation link if available
  • Short description
  • Example, named as operator example so that the heading generated can be linked to directly (i.e., Creating-Observables.md#justexample.

Related tracking issue: #6132

[codecov]

Codecov Report

Merging #6131 into 2.x will increase coverage by 0.03%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6131      +/-   ##
============================================
+ Coverage     98.23%   98.26%   +0.03%     
+ Complexity     6194     6191       -3     
============================================
  Files           667      667              
  Lines         44853    44853              
  Branches       6213     6213              
============================================
+ Hits          44063    44077      +14     
+ Misses          245      233      -12     
+ Partials        545      543       -2

Impacted Files	Coverage Δ	Complexity Δ
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	92.93% <0%> (-2.72%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.91% <0%> (-2.09%)	56% <0%> (-2%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-1.99%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	94.68% <0%> (-1.6%)	3% <0%> (ø)
...io/reactivex/subscribers/SerializedSubscriber.java	98.86% <0%> (-1.14%)	26% <0%> (-1%)
...perators/mixed/ObservableSwitchMapCompletable.java	98.94% <0%> (-1.06%)	3% <0%> (ø)
... and 18 more

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[vanniktech]

👍