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/guides/plugins/en.md

@@ -0,0 +1,25 @@
+---
+title: Plugin guide
+order: 400
+icons: 
+  - logo
+  - plugin
+for: developers
+about: |
+  This guide shows you everything you need to know to understand plugins in FreeSewing, and create your own.
+goals:
+ - Know about build-time plugins vs run-time plugins
+ - Understanding plugin structure
+ - Hooks and how to use them
+ - Using hooks without a plugin
+ - Using macros
+---
+
+Plugins allow you to extend FreeSewing.
+
+We have [a list of available plugins](/reference/plugins/), but
+if you can't find what you're looking for, you can write your own plugin.
+
+We'll cover the following topics in this guide:
+
+<ReadMore list />