CaptainAgent PR part 3

[LeoLjl]

In this PR, there are several milestones:

• Update the blogs according to the interface changes.
• Update the docs according to the interface changes.
• Add instructions about customizing user's own agent and tool library.
• Add the full tool library.

Why are these changes needed?

Related issue number

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]

Docstring could be improved. But otherwise fine.

[qingyun-wu]

TODO in a future PR: instead of putting all tools here, we should download/install agent/tools from a provided link. In this case, anyone can create their own tool/agent lib. As long the format is good, the lib should work together well with CaptainAgent.