ci: Define doc in .gitattributes

Example:

    git ls-files -- ':(attr:doc)'

Signed-off-by: Eunchan Kim <eunchan@opentitan.org>
diff --git a/.gitattributes b/.gitattributes
index 334a8b6..786d919 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -11,3 +11,18 @@
 hw/rdc/tools/*/run-rdc.tcl   cdc
 hw/rdc/tools/dvsim/rdc.mk    cdc dvsim
 hw/rdc/tools/dvsim/*.hjson   cdc dvsim
+
+# doc
+/doc/   doc
+**/*.md doc
+
+hw/**/doc/*.md         doc
+hw/**/doc/*.svg        doc
+hw/**/doc/*.png        doc
+hw/**/doc/*.jpg        doc
+hw/**/data/*.prj.hjson doc
+
+sw/**/*.md  doc
+sw/**/*.svg doc
+sw/**/*.jpg doc
+sw/**/*.png doc