Add multiple agents group assignment section

[GGP1]

Description

Closes #7205.

Adds a section to the Grouping agents document to explain how to assign multiple agents to a single group and then list the agents belonging to a group.

Pointed to 4.8.0 since it will be the new production branch soon. It mat require a rebase if it isn't merged in time.

Checks

Docs building

• Compiles without warnings.

Code formatting and web optimization

• Uses three spaces indentation.
• Adds or updates meta descriptions accordingly.
• Updates the redirects.js script if necessary (check this guide).

Writing style

• Uses present tense, active voice, and semi-formal registry.
• Uses short, simple sentences.
• Uses bold for user interface elements, italics for key terms or emphasis, and code font for Bash commands, file names, REST paths, and code.

[fdalmaup]

LGTM