Feat: creating promtail architecture document

Description

Creation of an Architecture.md document for Promtail.

Preview: https://repo1.dso.mil/platform-one/big-bang/bigbang/-/blob/promtail_arch_doc/docs/understanding-bigbang/package-architecture/promtail/Architecture.md

Issue

Closes https://repo1.dso.mil/platform-one/big-bang/apps/sandbox/promtail/-/issues/9