feat: Flat import of markdown repo

This is a flat (without history) import of (some of) the content
from our markdown module.

We've imported this without history because the repo contains our
blog posts and showcases posts content prior to porting them to strapi.

Since this contains many images, it would balloon the size of this repo
to import the full history.

Instead, please refer to the history of the (archived) markdown repo
at: https://github.com/freesewing/markdown

markdown/dev/reference/api/attributes/add/en.md

@@ -0,0 +1,31 @@
+---
+title: add()
+---
+
+```js
+Attributes attributes.add(string key, string value)
+```
+
+Adds `value` to the attribute identified by `key`.
+
+Adding multiple values to the same key will result in them being joined together 
+(with a space) when rendering.
+
+```js
+let { Path, paths } = part.shorthand();
+
+// This will render as: class="classA classB"
+paths.demo = new Path();
+paths.demo.attributes.add('class', 'classA');
+paths.demo.attributes.add('class', 'classB');
+
+// This does the same thing:
+paths.demo = new Path()
+  .attr('class', 'classA')
+  .attr('class', 'classB');
+
+// This also has the same result:
+paths.demo = new Path()
+  .attr('class', 'classA classB');
+```
+