docs: Add initial docs

docs/common-library/introduction.md

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+# Common library
+
+!!! note
+    The Common library chart is not meant to be installed directly, application
+    charts use the Common library as a dependency.
+
+## Background
+
+In Helm 3, their team introduced the concept of a
+[Library chart](https://helm.sh/docs/topics/library_charts/).
+
+> A library chart is a type of Helm chart that defines chart primitives or
+  definitions which can be shared by Helm templates in other charts. This
+  allows users to share snippets of code that can be re-used across charts,
+  avoiding repetition and keeping charts DRY.
+
+The common library was created because I saw many charts requiring only a
+few select configuration options in their Helm charts.
+
+!!! note
+    Take one of the many applications like `sonarr` or `nzbget`. Each of these
+    charts only require setting `service`, `port`, `persistence`, `ingress`
+    and `image` since state and app configuration is handled by the application
+    itself.
+
+In order to stay somewhat DRY (Don't Repeat Yourself) and keeping with Helm 3
+usage for a Library chart, I saw this pattern and decided it was worth it to
+create a library. This means each one of these app charts has a
+dependency on what we call the `common` library.
+
+## Source code
+
+The source code for the common library chart can be found
+[here](https://github.com/bjw-s/helm-charts/tree/main/charts/library/common).