docs: add warning about running sq-poller in background

[LucaNicosia]

Description

Adds a warning the sq-poller page to inform the user how to start the poller in background.

Type of change

• Documentation update

Double Check

• [x] I have read the comments and followed the CONTRIBUTING.md.
• [x] I have explained my PR according to the information in the comments or in a linked issue.
• [x] My PR source branch is created from the develop branch.
• [x] My PR targets the develop branch.
• [x] All my commits have --signoff applied