feature(docs): Document how functions work in plugins/themes (#33263)

* feature(docs): Document how functions work in plugins/themes

* Update docs/docs/reference/functions/getting-started.md

Co-authored-by: Megan Sullivan <megan@gatsbyjs.com>

* Update docs/docs/reference/functions/getting-started.md

Co-authored-by: Megan Sullivan <megan@gatsbyjs.com>

* Update getting-started.md

* Update getting-started.md

* Update getting-started.md

Co-authored-by: Megan Sullivan <megan@gatsbyjs.com>

docs/docs/reference/functions/getting-started.md

@@ -190,6 +190,16 @@ export default function FormPage() {
 }
 ```
 
+## Functions in plugins and themes
+
+Plugins and themes can ship functions! This is powerful as it lets you pair frontend code with backend code. For example, if you built a plugin for an authorization service that includes a login component, you could ship alongside the component, a serverless function the component can use to connect to the remote API.
+
+### Namespacing
+
+Plugin/theme functions work exactly the same as normal functions except their routes must be created under the plugin's namespace e.g. `${PLUGIN_ROOT}/src/api/{pluginName}/my-plugin-function.js`.
+
+Shadowing with functions works similar to how shadowing works in general. You can shadow a plugin/theme function by copying the file from the plugin/theme's `src` tree into your site's `src` tree. For example, to shadow the `/gatsby-plugin-cool/do-something` function from the `gatsby-plugin-cool` plugin, you'd copy `node_modules/gatsby-plugin-cool/src/api/gatsby-plugin-cool/do-something.js` to `src/api/gatsby-plugin-cool/do-something.js`. From there, you can overwrite the implementation of the `/do-something` function as you normally would.
+
 ## Limitations
 
 - Gatsby Functions do not support dynamic routes in Gatsby Cloud at the moment