Improve "Agent propagation" explanation

## Edit existing documentation 📝

### Page to edit
/reference/agent-propagation

### Required changes

This page is really **Explanation**, not **Reference**. Inaccurate statements should be removed. More detail should be given about how/why the agent is self-propagating. I recommend moving this page to /features/ and renaming it "Self-propagating Agent" or something similar.

Address https://github.com/guardicore/monkey/pull/4240#issuecomment-2231838998, consider adding a separate section about available credentials for exploiters (configured credentials, as well as those stolen from other machines) and update individual brute-force exploiters' pages to link to this section instead of mentioning "configured and stolen" wherever credential usage is mentioned