[Docs] Rewrite Swarms Documentation

[harishmohanraj]

The current documentation is unclear and lengthy. We need to rewrite it to provide a clear explanation of the concepts before diving into the details. For example:

• Explain the concept of swarms: Provide a clear and concise definition of what swarms are, their purpose, and how they function.

• Differentiate between swarms and GroupChat: Clarify the key differences between swarms and GroupChat, and outline the scenarios where each should be used.

• Explain the concept of context variables: Clearly define what context variables are, their role, and how they are utilized within the system.

[marklysze]

CC @yiranwu0