ci: run full pipeline on docs-only changes

This is technically more heavy than necessary, but I think changes
that only touch docs won't be too frequent and it's better than
no CI at all on those PRs.

Fixes #796.

.github/workflows/native-unix.yml

@@ -25,6 +25,7 @@ on:
       - "adbc.h"
       - "c/**"
       - "ci/**"
+      - "docs/**"
       - "glib/**"
       - "go/**"
       - "python/**"
@@ -36,6 +37,7 @@ on:
       - "adbc.h"
       - "c/**"
       - "ci/**"
+      - "docs/**"
       - "glib/**"
       - "go/**"
       - "python/**"