Configure kotlindoc generation per SDK.

[vkryachko]

This makes it easier to generate docs for a subset of SDKs, additionally
it produces a set of separate _toc.yaml files as opposed to a global one.

[codecov]

Codecov Report

Merging #1106 into master will decrease coverage by 4.13%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1106      +/-   ##
============================================
- Coverage     57.56%   53.42%   -4.14%     
- Complexity     6238     6408     +170     
============================================
  Files           639      719      +80     
  Lines         31367    34853    +3486     
  Branches       4323     4624     +301     
============================================
+ Hits          18055    18620     +565     
- Misses        11870    14758    +2888     
- Partials       1442     1475      +33

Impacted Files	Coverage Δ	Complexity Δ
...le/firebase/firestore/util/ExponentialBackoff.java	28.26% <0%> (-0.32%)	3% <0%> (ø)
...va/com/google/firebase/database/core/SyncTree.java	62% <0%> (-0.25%)	40% <0%> (ø)
...le/firebase/firestore/remote/GrpcCallProvider.java	45.45% <0%> (-0.16%)	8% <0%> (+4%)
...ions/remote/FirebaseInstallationServiceClient.java	6.66% <0%> (-0.1%)	3% <0%> (ø)
...tics/internal/stacktrace/TrimmedThrowableData.java	0% <0%> (ø)	0% <0%> (?)
...ase/crashlytics/internal/report/ReportManager.java	0% <0%> (ø)	0% <0%> (?)
...e/crashlytics/internal/ndk/ProcMapEntryParser.java	0% <0%> (ø)	0% <0%> (?)
...rashlytics/internal/ndk/BinaryImagesConverter.java	0% <0%> (ø)	0% <0%> (?)
...ytics/internal/settings/model/AppSettingsData.java	0% <0%> (ø)	0% <0%> (?)
...oogle/firebase/crashlytics/core/MetaDataStore.java	0% <0%> (ø)	0% <0%> (?)
... and 93 more

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[rlazo]

/retest

[rlazo]

/test copyright-check