Fine-tune GitHub's language detection for the repo

Since the whole repo is dedicated to documentation, documentation is the "source code" of this repo.
In order to capture this in language stats (and potentially attract more contributors):
1. Consider `docs/` folder "source", not documentation.
2. Count all Markdown files as "source".

.gitattributes

@@ -4,3 +4,7 @@
 *.jpg filter=lfs diff=lfs merge=lfs -text
 *.png filter=lfs diff=lfs merge=lfs -text
 *.md diff=markdown
+
+# Fine-tune GitHub's language detection
+docs/** -linguist-documentation
+*.md linguist-detectable