[Docs] Communication Tools and Agents documentation

[marklysze]

Why are these changes needed?

Documentation and blog post for the communication tools and agents for Discord, Slack, and Telegram.

For Issue #442
Depends on merging of #754 and #787 and #835

Related issue number

N/A

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/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.