2.x: Update _Sidebar.md with new order of topics

[akarnokd]

Change the sidebar to support the v2 features and suggest new topic orderings.

Related tracking issue: #6132

[codecov]

Codecov Report

Merging #6133 into 2.x will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                2.x    #6133      +/-   ##
============================================
+ Coverage     98.23%   98.29%   +0.05%     
  Complexity     6194     6194              
============================================
  Files           667      667              
  Lines         44853    44853              
  Branches       6213     6213              
============================================
+ Hits          44063    44089      +26     
+ Misses          245      230      -15     
+ Partials        545      534      -11

Impacted Files	Coverage Δ	Complexity Δ
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...reactivex/internal/operators/single/SingleAmb.java	96.36% <0%> (-3.64%)	9% <0%> (-1%)
...a/io/reactivex/internal/util/QueueDrainHelper.java	97.22% <0%> (-2.78%)	56% <0%> (-2%)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...rnal/operators/observable/ObservableSwitchMap.java	95.21% <0%> (-1.07%)	3% <0%> (ø)
...ternal/operators/flowable/FlowableWindowTimed.java	91.17% <0%> (-0.68%)	4% <0%> (ø)
...l/operators/observable/ObservableFlatMapMaybe.java	90.19% <0%> (-0.66%)	2% <0%> (ø)
...ernal/operators/flowable/FlowableFromIterable.java	95.18% <0%> (-0.54%)	5% <0%> (ø)
...vex/internal/operators/parallel/ParallelRunOn.java	96.61% <0%> (-0.49%)	8% <0%> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	97.41% <0%> (ø)	6% <0%> (ø)	⬇️
... and 13 more

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[artem-zinnatullin]

phew, gladly "Async" is alphabetically earlier than "Blocking"

[vanniktech]

👍