[DOCS]: Add documentation explaining how to create and use "Common Credentials"

[psanders]

Description:

Common Credentials is a new feature introduced in Routr v2 that enables the creation of shared credentials sets with usernames and passwords. These credentials can then be assigned to multiple agents, peers, or trunks, simplifying device authentication management, especially for large numbers of IoT devices where individual configuration is impractical.

Use Cases:

• Assign common credentials to all door phones on the network for centralized authentication.
• Simplify the setup of care alarms by using shared credentials across multiple devices.
• Any scenario where managing individual device credentials is cumbersome or impractical.

This ticket is to enhance the documentation by explaining the use-case and the "how to" of the Common Credentials feature.