docs: add the plugin introduction guide

Author: fahchen

guides/plugins/introduction.md

@@ -0,0 +1,17 @@
+# Introduction to the plugin system
+
+For more customization, `TypedStructor` provides a plugin system
+that allows you to extend the functionality of the library.
+This is useful when you want to extract some common logic into a separate module.
+
+See `TypedStructor.Plugin` for how to create a plugin.
+
+This library comes with a few built-in plugins, and we don't like to
+implement more built-in plugins, but instead, we encourage you to create your own plugins.
+We provide some example plugins that you can use as a reference, or copy-paste.
+
+**Plugin examples:**
+- [Registering plugins globally](./registering_plugins_globally.md)
+- `TypedStructor.Plugins.Accessible`
+- [Type Only on Ecto Schema](./type_only_on_ecto_schema.md)
+- [Add primary key and timestamps types to your Ecto schema](./primary_key_and_timestamps.md)

mix.exs

@@ -31,6 +31,7 @@ defmodule TypedStructor.MixProject do
           "guides/migrate_from_typed_struct.md",
 
           # plugins
+          {"guides/plugins/introduction.md", [title: "Introduction"]},
           "guides/plugins/registering_plugins_globally.md",
           "guides/plugins/type_only_on_ecto_schema.md",
           "guides/plugins/primary_key_and_timestamps.md"