Add generate examples to Creating-Observables.md in Wiki

[elijahverdoorn]

Add documentation and example to the wiki for generate.

Resolves: #6226

[codecov]

Codecov Report

Merging #6260 into 2.x will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##                2.x    #6260   +/-   ##
=========================================
  Coverage     98.22%   98.22%           
  Complexity     6203     6203           
=========================================
  Files           667      667           
  Lines         44889    44889           
  Branches       6216     6216           
=========================================
  Hits          44093    44093           
+ Misses          254      250    -4     
- Partials        542      546    +4

Impacted Files	Coverage Δ	Complexity Δ
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...a/io/reactivex/internal/util/QueueDrainHelper.java	96.52% <0%> (-3.48%)	55% <0%> (-3%)
...rnal/operators/flowable/FlowableFlatMapSingle.java	93.47% <0%> (-2.18%)	2% <0%> (ø)
...internal/operators/flowable/FlowableSwitchMap.java	95.28% <0%> (-1.89%)	3% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	97.87% <0%> (-1.71%)	11% <0%> (ø)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-1.07%)	5% <0%> (ø)
...l/operators/observable/ObservableTimeoutTimed.java	99.15% <0%> (-0.85%)	3% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	97.5% <0%> (-0.84%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowableFlatMap.java	88.94% <0%> (-0.79%)	4% <0%> (ø)
... and 20 more

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