Fix linguist tool configuration (exclude docs from stats) (#1466)

Summary:
With this change, language stats will be without html:

![image](https://user-images.githubusercontent.com/19646569/124871281-0ab86800-dfee-11eb-9551-7465394bc556.png)

Pull Request resolved: https://github.com/facebook/infer/pull/1466

Reviewed By: ngorogiannis

Differential Revision: D29612707

Pulled By: jvillard

fbshipit-source-id: c7ce68046

.gitattributes

@@ -1 +1 @@
-website/* linguist-documentation
+website/** linguist-documentation