Add javadoc for 0.13.0

[samredai]

This adds the javadoc for 0.13.0. I generated this by cloning apache/iceberg and running:

git checkout 0.13.x
./gradlew refreshJavadoc

Then I replaced the javadoc directory in apache/iceberg-docs with the contents of the javadoc generated for 0.13.0:

rm -rf iceberg-docs/javadoc
cp -r iceberg/site/docs/javadoc/0.13.0 iceberg-docs/javadoc

[samredai]

@jackye1995

[jackye1995]

Thanks! There is a difference between the javadoc folder structure generated between me and Sam, we have synced offline. Given the fact that what Sam generated follows the structure of the old versions, we decided to use Sam's version. I am investigating what is causing the difference and will post result later.