Advertise Markdown files in website/content/ as "source" files (#221)

According to README:
> The source pages are written in Markdown format under the
`website/content` folder.

This change aims to reflect that in language stats:
|Before|After|
|---|---|

|![image](https://github.com/user-attachments/assets/b81f7ce1-b15f-4f90-aaf3-a6e1e304a081)|![image](https://github.com/user-attachments/assets/21a1c747-a5bd-4d48-b506-ab589592ebca)|

|![image](https://github.com/user-attachments/assets/f6c097c7-d3e6-4b20-a05c-43892a9141e4)|![image](https://github.com/user-attachments/assets/08e5d50b-7029-4879-9d94-9aec5974cf68)|

In my opinion, properly detecting Markdown as the dominant language of
the repository will help people (e.g. documentation writers and beginner
programmers) to find a project to contribute to.

Author: i-ky

.gitattributes

@@ -0,0 +1 @@
+website/content/**/*.md linguist-detectable