Merge branch 'promtail_arch_doc' into 'master'

Feat: creating promtail architecture document

Closes platform-one/big-bang/apps/sandbox/promtail#9

See merge request platform-one/big-bang/bigbang!1969

docs/understanding-bigbang/package-architecture/promtail/Architecture.md

+# Promtail
+
+## Overview
+
+> Promtail is an agent which reads log files and sends streams of log data to the centralised Loki instances along with a set of labels.
+
+## Big Bang Touch Points
+
+```mermaid
+graph TB
+
+  subgraph "Promtail"
+    promtail
+  end      
+
+  subgraph "Loki"
+    promtail --> loki
+  end
+
+```
+
+### Architecture: 
+- [Promtail Client](https://grafana.com/docs/loki/latest/clients/promtail/)
+
+### Storage
+
+Promtail does not persist data, and instead reads log files and streams the data to a log aggregation system.
+
+### Istio Configuration
+
+Istio is disabled in the promtail chart by default and can be enabled by setting the following values in the bigbang chart:
+
+```yaml
+istio:
+  enabled: true
+```
+
+These values get passed into the promtail chart [here](https://repo1.dso.mil/platform-one/big-bang/apps/sandbox/promtail/-/blob/main/chart/values.yaml#L428). 
+
+## High Availability
+
+By default, Promtail runs as a `daemonset` with a pod on each node.
+
+## Single Sign on (SSO)
+
+None. This service doesn't have a web interface.
+
+
+## Licensing
+
+Promtail utilizes an [AGPLv3 License](https://github.com/grafana/loki/blob/main/LICENSE) for it's code and binaries.
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