RATIS-1575. Handle hugo lock file

[adoroszlai]

What changes were proposed in this pull request?

Exclude Hugo's lock file from source tarball, and let Rat and Git ignore it.

https://issues.apache.org/jira/browse/RATIS-1575

How was this patch tested?

$ mvn -DskipTests -Prelease -Papache-release clean package assembly:single 
...
[INFO] BUILD SUCCESS

$ ls -s ratis-docs/.hugo_build.lock
0 ratis-docs/.hugo_build.lock

$ dev-support/checks/rat.sh
...
[INFO] BUILD SUCCESS

$ tar tzvf ratis-assembly/target/apache-ratis-*-src.tar.gz| grep hugo | wc -l
       0

$ git status
On branch RATIS-1575

nothing to commit, working tree clean

[szetszwo]

+1 the change looks good.

[adoroszlai]

Thanks @szetszwo for reviewing and committing it.