feat: exclude markdown files in src/ from linguist documentation

Add .gitattributes file to explicitly mark markdown files in the
src/ directory as non-documentation. This prevents GitHub Linguist
from treating them as project documentation, which is useful when
markdown files in src/ are used for other purposes (e.g., code
comments, templates, or non-user-facing content).

.gitattributes

@@ -0,0 +1,3 @@
+# Do not consider Markdown files documentation anymore for
+# the Documentation!
+src/**/*.md -linguist-documentation