new: added notification of Err creations, etc

[sttk]

Changes:

1. cicd: modified javadoc publish dir

   Because maven outputs javadoc to ./target/site/apidocs, I changed source directory to publish to github pages in github actions configuration file.

2. chore: added clean, jar and default tasks to build.sh

   I adds 3 tasks to clean this project, make jar file, and execute some build tasks in a lump.

3. new: added notification of Err creations

   I adds ErrHandler and ErrNotifier classes to notify Err object creations.

[sttk]

There is 1 issue I pointed out. Take actions for it.

[sttk]

Approved.