feat(markdown): Ported code howtos to v3

markdown/dev/howtos/code/adding-parts/en.md

@@ -1,42 +1,29 @@
 ---
 title: Adding pattern parts
-for: developers
-about: Shows you how to add new parts to your pattern
 ---
 
-Since the patterns parts are listed
-in [the configuration file](/reference/api/config/), freesewing knows about
-all the parts that belong to your pattern.
+Parts can be added to the design add build time, by passing them to [the Design
+constructor](/reference/api/design), or at runtime by calling
+[Pattern.addPart()](/reference/api/pattern/addpart). The latter approach is
+rarely used, but it's there if you need it.
 
-It expects that each pattern has its own draft method, that is called `draft`
-followed by the capitalized name of the pattern part.
+## At build time
 
-For example, if our pattern `Sorcha` has a part called `back`, you should
-have a `draftBack` method. It's good practice to keep each part in its own
-file, so create a file called `back.js`. Inside, you export your method
-to draft this part:
+```mjs
+import { Design } from '@freesewing/core'
+import { myPart } from './mypart.mjs'
 
-```js
-export default part => {
-  // Your part code here
-
-  return part
-}
+const Sorcha = new Design({
+  parts: [ myPart ]
+})
 ```
 
-Then, in your `index.js` file, you import this file, and attach the
-method to your pattern's prototype:
+## At run time
 
-```js
-import freesewing from "freesewing"
-import plugins from "@freesewing/plugin-bundle"
-import config from "../config"
-// Parts
-import draftBack from "./back"
+```mjs
+import { Aaron } from '@freesewing/aaron'
+import { myRuntimePart } from './mypart.mjs'
 
-// Create new design
-const Sorcha = new freesewing.Design(config, plugins)
-
-// Attach to pattern prototype
-Sorcha.prototype.draftBack = part => draftBack(part)
+const pattern = new Aaron()
+pattern.addPart(myRuntimePart)
 ```