automatically create latest/ symlink in docs for tagged builds

[nfriedly]

Summary

This is to answer the docs team's request to be able to link to the latest release's javadoc without continually having to update things. It should also make life easier on us.

Other Information

This actually points to the latest tag, so if you, say, release v3.0.0 and then go back and publish v2.9.1 with a bugfix, the latest/ link will change to v2.9.1 until you release a new v3.x. (Or you change it manually in the gh-pages branch.)

I'm sending this as a PR rather than committing it so that @germanattanasio can review it and choose when to test it on a new release.

[codecov-io]

Current coverage is 70.05%

Merging #222 into master will decrease coverage by -0.04% as of ead7f3c

@@            master    #222   diff @@
======================================
  Files           67      67       
  Stmts         2284    2284       
  Branches       420     420       
  Methods          0       0       
======================================
- Hit           1601    1600     -1
- Partial        240     241     +1
  Missed         443     443       

Review entire Coverage Diff as of ead7f3c

Powered by Codecov. Updated on successful CI builds.

[nfriedly]

@germanattanasio can you do a version bump to validate this? Also, I see a latest-docs-symlink folder which is actually for this branch... we should delete that.