docs: backfill next readme with getGlobalStyles() and getTemplateData()

docs/mutating-query.md

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+# Global Styles
+
+Global styles can be mutated by passing `getGlobalStyles` attribute to `RootLayout` in `src/app/layout.tsx`.
+
+`getGlobalStyles` takes an async callback function that returns an object containing global styles.
+
+The default definition for `getGlobalStyles` function passed in `getGlobalStyles` attribute to `RootLayout` in `src/app/[[...path]]/layout.tsx`, can be found in [@snapwp/query](../packages/query/README.md) package.
+
+Type Definition of `getGlobalStyles`:
+
+```typescript
+type getGlobalStyles = () => Promise< GlobalHeadProps >;
+```
+
+Type definition of `GlobalHeadProps` can be found in [@snapwp/core](../packages/core/README.md) package.
+
+# Template Data
+
+Template data can be mutated by passing `getTemplateData` attribute to `TemplateRenderer` in `src/app/[[...path]]/page.tsx`.
+
+`getTemplateData` takes an async callback to get template styles and content.
+
+The default definition for `getTemplateData` function passed in `getTemplateData` attribute to `TemplateRenderer` in `src/app/[[...path]]/page.tsx`, can be found in [@snapwp/query](../packages/query/README.md) package.
+
+Type Definition of `getTemplateData`:
+
+```typescript
+type getTemplateData = ( uri: string ) => Promise< TemplateData >;
+```
+
+Type definition of `TemplateData` can be found in [@snapwp/core](../packages/core/README.md) package.