License checks and updates

[marklysze]

Why are these changes needed?

To assist developers, I've added a "check licence headers" check in pre-commit that will check all python files for the copyright header.

In doing so, a number of existing python files required updates.

Documentation also required an update.

Related issue number

N/A

Checks

• I've included any doc changes needed for https://ag2ai.github.io/ag2/. See https://ag2ai.github.io/ag2/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[qingyun-wu]

LGTM. Thanks Mark!