fix: Scaling content on Learn page

This commit fixes an issue on the `/learn` page introduced with
the mindmap for scaling.

Acording to my tests:
1. If a folder does not have a `_index.md` file, every file under
   the folder will generate a section under the `/learn` page. in
   This way, both files, `mindmap.md` and `markmap_content.md`
   were generating content. To fix this, I renamed `mindmap.md`
   to `_index.md`.
2. When a file does not mave a `description` property in the
   frontmatter block, it will use the content of the file as
   description. To fix this I added a `description` property to
   `mindmap.md` (now renamed to `_index.md`.

Related issues
- #902 (comment)

Author: Guilherme Dellagustin

content/en/learn/scaling/mindmap.md content/en/learn/scaling/_index.mdcontent/en/learn/scaling/mindmap.md renamed to content/en/learn/scaling/_index.md

@@ -1,6 +1,7 @@
 ---
 title: "Scaling InnerSource"
 subtitle: "How can I scale InnerSource?"
+description: "A mindmap to help you use different parts of the InnerSource Commons material to scale InnerSource"
 type: "community"
 image: "images/learn/patterns.png"
 weight: 1