chore(markdown): Restructuring dev docs

markdown/dev/reference/api/config/inject/en.md

@@ -0,0 +1,20 @@
+---
+title: inject
+---
+
+```js
+inject: {
+  front: "back"
+}
+```
+
+An object of key/value pairs of parts. The `value` part will be injected in the `key` part.
+
+By *injected* we mean rather than starting out with a fresh part, you'll get a part that
+has the points, paths, and snippets of the `value` part.
+
+<Tip>
+
+See [the Howto on Part inheritance](/howtos/code/inject) for an example.
+
+</Tip>