chore: Port FreeSewing.dev to docusaurus

The replaces the NextJS site powering FreeSewing.dev with a Docusaurus
setup. It's part of my efforts to simplify FreeSewing's setup so we can
focus on our core value proposition.

sites/dev/docs/howtos/code/adding-parts/readme.mdx

@@ -0,0 +1,29 @@
+---
+title: Adding pattern parts
+---
+
+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.
+
+## At build time
+
+```mjs
+import { Design } from '@freesewing/core'
+import { myPart } from './mypart.mjs'
+
+const Sorcha = new Design({
+  parts: [ myPart ]
+})
+```
+
+## At run time
+
+```mjs
+import { Aaron } from '@freesewing/aaron'
+import { myRuntimePart } from './mypart.mjs'
+
+const pattern = new Aaron()
+pattern.addPart(myRuntimePart)
+```