Fix temp. directories created by `update-gh-pages` script

[alexander-yevsyukov]

The script creates temporary directories prefixed with javadoc and repoTemp.
These directories don't seem to be deleted ever.

We should update the script (hopefully migrating it to Kotlin), so that the temporary directories are removed when JVM shuts down. Some efforts on this front are made by this PR in base. Please look into it, it may hit on steps needed to address this issue.