Added Javadoc checks to Checkstyle. Fix violating Javadoc.

[benjamintboyle]

Add JavadocMethod and MissingJavadocMethod checks to Checkstyle.
Checks ensure there are no missing Javadoc comments for public methods,
and that each of the Javadoc comments describe all method parameters.

Add suppressions.xml Checkstyle configuration file to exclude Jmh, Test,
and Internal from MissingJavadocMethod check.

Move Checkstyle configuration files and License headers into config directory, to reduce pollution of root directory.

Fix Javadoc comments that violate the added checks, which should take care of the first bullet from Issue #4535 (parameter names change in the code but the javadocs still refer to the old names).

[codecov]

Codecov Report

Merging #7210 (ca0e0eb) into 3.x (26dddf5) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##                3.x    #7210      +/-   ##
============================================
- Coverage     99.57%   99.55%   -0.02%     
- Complexity     6758     6759       +1     
============================================
  Files           747      747              
  Lines         47392    47392              
  Branches       6382     6382              
============================================
- Hits          47189    47180       -9     
- Misses           95       99       +4     
- Partials        108      113       +5     

Impacted Files	Coverage Δ	Complexity Δ
...in/java/io/reactivex/rxjava3/core/Completable.java	100.00% <ø> (ø)	140.00 <0.00> (ø)
.../main/java/io/reactivex/rxjava3/core/Flowable.java	100.00% <ø> (ø)	597.00 <0.00> (ø)
...n/java/io/reactivex/rxjava3/core/Notification.java	100.00% <ø> (ø)	23.00 <0.00> (ø)
...ain/java/io/reactivex/rxjava3/core/Observable.java	100.00% <ø> (ø)	565.00 <0.00> (ø)
...activex/rxjava3/exceptions/CompositeException.java	100.00% <ø> (ø)	35.00 <0.00> (ø)
...a3/internal/operators/flowable/FlowableReplay.java	98.92% <ø> (-0.86%)	21.00 <0.00> (ø)
...a3/internal/operators/mixed/ScalarXMapZHelper.java	100.00% <ø> (ø)	13.00 <0.00> (ø)
...nternal/operators/observable/ObservableReplay.java	99.46% <ø> (-0.54%)	22.00 <0.00> (ø)
...ctivex/rxjava3/internal/util/QueueDrainHelper.java	100.00% <ø> (ø)	58.00 <0.00> (ø)
.../reactivex/rxjava3/observers/BaseTestConsumer.java	100.00% <ø> (ø)	75.00 <0.00> (ø)
... and 10 more

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