ci: add API docs workflow

[mtrezza]

javadoc generation when pushing to master branch

related: #1059

[mtrezza]

Locally javadoc gen currently fails with:

> Task :gcm:generateReleaseRFile FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':gcm:generateReleaseRFile'.
> Could not resolve all files for configuration ':gcm:releaseCompileClasspath'.
   > Could not find com.firebase:firebase-jobdispatcher:0.8.6.
     Required by:
         project :gcm

But it builds after removing GCM module; let's try again after #1091

[codecov]

Codecov Report

Merging #1110 (9060169) into master (aa16bcf) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #1110   +/-   ##
=========================================
  Coverage     65.06%   65.06%           
  Complexity     2208     2208           
=========================================
  Files           122      122           
  Lines          9463     9463           
  Branches       1317     1317           
=========================================
  Hits           6157     6157           
  Misses         2799     2799           
  Partials        507      507           

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[azlekov]

@mtrezza

> Task :gcm:generateReleaseRFile FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':gcm:generateReleaseRFile'.
> Could not resolve all files for configuration ':gcm:releaseCompileClasspath'.
   > Could not find com.firebase:firebase-jobdispatcher:0.8.6.
     Required by:
         project :gcm

Failed because this dependency is available only on the deprecated JCenter repository. You can bring it back in order to have this proceed. https://mvnrepository.com/artifact/com.firebase/firebase-jobdispatcher/0.8.6

Just add:

jcenter() to the repositories blocks inside the root build.gradle

[mtrezza]

It works locally after removing the GCM, so I‘ll just wait for the other PR to get merged.