3.x: Javadocs package-info made nicer, update README regarding Java 8

[akarnokd]

• Update README.md and indicate Java 8 requirement.
• Enable @Nullable to be put on type parameters and type uses.
• Add links to components in various package-info.java documentations for easier navigation in the JavaDocs summary.

[codecov]

Codecov Report

Merging #6778 into 3.x will increase coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##               3.x    #6778      +/-   ##
===========================================
+ Coverage     98.2%   98.21%   +0.01%     
- Complexity    6187     6189       +2     
===========================================
  Files          681      681              
  Lines        44776    44776              
  Branches      6164     6164              
===========================================
+ Hits         43971    43976       +5     
  Misses         281      281              
+ Partials       524      519       -5

Impacted Files	Coverage Δ	Complexity Δ
...l/operators/observable/ObservableFlatMapMaybe.java	89.43% <0%> (-2.82%)	2% <0%> (ø)
...perators/single/SingleFlatMapIterableFlowable.java	94.16% <0%> (-2.5%)	2% <0%> (ø)
...eactivex/rxjava3/processors/BehaviorProcessor.java	96.58% <0%> (-2.44%)	51% <0%> (ø)
...nternal/operators/parallel/ParallelSortedJoin.java	92.7% <0%> (-2.19%)	2% <0%> (ø)
...rnal/operators/observable/ObservableSwitchMap.java	91.57% <0%> (-1.58%)	3% <0%> (ø)
...va3/internal/operators/flowable/FlowableCache.java	97.72% <0%> (-0.76%)	37% <0%> (-1%)
...3/internal/operators/flowable/FlowableGroupBy.java	95.28% <0%> (-0.63%)	3% <0%> (ø)
...internal/operators/flowable/FlowableObserveOn.java	96.65% <0%> (-0.56%)	3% <0%> (ø)
...ternal/operators/completable/CompletableMerge.java	97.29% <0%> (ø)	2% <0%> (ø)	⬇️
...a3/internal/operators/flowable/FlowableCreate.java	97.41% <0%> (+0.32%)	6% <0%> (ø)	⬇️
... and 10 more

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