Add example for custom schema setings

docs/_includes/examples/custom_settings.rs

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+use schemars::{gen::SchemaSettings, JsonSchema};
+
+#[derive(JsonSchema)]
+pub struct MyStruct {
+    pub my_int: i32,
+    pub my_bool: bool,
+    pub my_nullable_enum: Option<MyEnum>,
+}
+
+#[derive(JsonSchema)]
+pub enum MyEnum {    
+    StringNewType(String),
+    StructVariant {
+        floats: Vec<f32>,
+    }
+}
+
+fn main() {
+    let settings = SchemaSettings {
+        option_nullable: true,
+        option_add_null_type: false,
+        ..SchemaSettings::draft07()
+    };
+    let gen = settings.into_generator();
+    let schema = gen.into_root_schema_for::<MyStruct>();;
+    println!("{}", serde_json::to_string_pretty(&schema).unwrap());
+}

docs/_includes/examples/custom_settings.schema.json

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+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "title": "MyStruct",
+  "type": "object",
+  "required": [
+    "my_bool",
+    "my_int",
+    "my_nullable_enum"
+  ],
+  "properties": {
+    "my_bool": {
+      "type": "boolean"
+    },
+    "my_int": {
+      "type": "integer",
+      "format": "int32"
+    },
+    "my_nullable_enum": {
+      "allOf": [
+        {
+          "$ref": "#/definitions/MyEnum"
+        }
+      ],
+      "nullable": true
+    }
+  },
+  "definitions": {
+    "MyEnum": {
+      "anyOf": [
+        {
+          "type": "object",
+          "required": [
+            "StringNewType"
+          ],
+          "properties": {
+            "StringNewType": {
+              "type": "string"
+            }
+          }
+        },
+        {
+          "type": "object",
+          "required": [
+            "StructVariant"
+          ],
+          "properties": {
+            "StructVariant": {
+              "type": "object",
+              "required": [
+                "floats"
+              ],
+              "properties": {
+                "floats": {
+                  "type": "array",
+                  "items": {
+                    "type": "number",
+                    "format": "float"
+                  }
+                }
+              }
+            }
+          }
+        }
+      ]
+    }
+  }
+}

docs/examples/3-custom_settings.md

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+---
+layout: default
+title: Custom Schema Settings
+parent: Examples
+nav_order: 3
+summary: Generating a schema using custom settings which changes how Option<T> is handled.
+---
+
+# Custom Schema Settings
+
+The `gen` module allows you to customise how schemas are generated. For example, the default behaviour for `Option<T>` is to include `null` in the schema's `type`s, but we can instead add a `nullable` property to its schema:
+
+{% include example.md name="custom_settings" %}