[docs] PAMSignal architecture document

[anhtuank7c]

Need to explain What/Why/When/How PAMSignal works.

• Overview of Linux Architecture.
• What is PAM (Pluggable Authentication Module)?
• What is systemd, journal?
• Compare the previous solution with current decision (recurringly scan auth.log vs systemd, journal)?
• How PAMSignal works under the hood?

[anhtuank7c]

Write to this document: https://github.com/anhtuank7c/pamsignal/blob/main/docs/phase1-systemd-jounal.md