Include multiple agent assignment to the same group documentation section

[Rebits]

Description

Include in Grouping agents documentation page a section about assigning multiple agents to the same group.

As @Selutario commented here, this is possible using the following commands:

• Create a group

curl -k -X POST "https://localhost:55000/groups?pretty=true" -H  "Authorization: Bearer $TOKEN" -H  "Content-Type: application/json" -d "{\"group_id\":\"aws_agents\"}"

• Assign multiple agents to the same group

curl -k -X PUT "https://localhost:55000/agents/group?agents_list=005,004&group_id=aws_agents" -H  "Authorization: Bearer $TOKEN"

[GGP1]

Update

Added a new section indicating how to add multiple agents to a single group and how to list the agents belonging to a group.