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) count Markdown files in content/ folder as "source".

.gitattributes

@@ -1,2 +1,5 @@
 # Auto-detect text files, ensure they use LF.
 * text=auto eol=lf
+
+# Fine-tune GitHub's language detection
+content/**/*.md linguist-detectable