Docs: addition to document systemd startup and fs_cli usage.

[DaggieBlanqx]

This addition documents how to start FreeSWITCH using systemd after installation and how to connect to the FreeSWITCH CLI. The reason is that the service does not start automatically, accessing the event socket requires explicitly starting the service.

Description

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code cleanup / refactor

Related Issues

Testing

• Added/updated unit tests
• Tested manually
• Tested with live SignalWire credentials (if applicable)

Checklist

• I have read the CONTRIBUTING guidelines
• My code follows the project's style guidelines
• I have added tests for my changes (if applicable)
• I have updated documentation (if applicable)
• All existing tests pass

Additional Notes

While installing on Debian 12 (bookworm) on multiple servers, I came across the issue of fs_cli -rRS failing to connect the socket, then I figured that it is because Freeswitch doesn't actually start automatically.
Hence, I am adding this small fix to the documentation to make it explicit.