docs(www): Update links to tutorials in plugins overview (#12466)

Author: phred

docs/docs/how-plugins-work.md

@@ -22,9 +22,10 @@ There are four standard plugin naming conventions for Gatsby:
 
 - **`gatsby-source-*`** — a source plugin loads data from a given source (e.g. WordPress, MongoDB, the file system). Use this plugin type if you are connecting a new source of data to Gatsby.
   - Example: [`gatsby-source-contentful`](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-source-contentful)
-  - Docs: [create a source plugin](/docs/create-source-plugin/)
+  - Docs: [creating a source plugin](/docs/creating-a-source-plugin/)
 - **`gatsby-transformer-*`** — a transformer plugin converts data from one format (e.g. CSV, YAML) to a JavaScript object. Use this naming convention if your plugin will be transforming data from one format to another.
   - Example: [`gatsby-transformer-yaml`](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-transformer-yaml)
+  - Docs: [creating a transformer plugin](/docs/creating-a-transformer-plugin/)
 - **`gatsby-[plugin-name]-*`** — if a plugin is a plugin for another plugin 😅, it should be prefixed with the name of the plugin it extends (e.g. if it adds emoji to the output of `gatsby-transformer-remark`, call it `gatsby-remark-add-emoji`). Use this naming convention whenever your plugin will be included as a plugin in the `options` object of another plugin.
   - Example: [`gatsby-remark-images`](https://github.com/gatsbyjs/gatsby/tree/master/packages/gatsby-remark-images)
 - **`gatsby-plugin-*`** — this is the most general plugin type. Use this naming convention if your plugin doesn’t meet the requirements of any other plugin types.