netlify: Only trigger preview when there are changes in docs. (#10144)

Signed-off-by: Jintao Zhang <[email protected]>

Author: tao12345666333

netlify.toml

@@ -2,6 +2,7 @@
 [build]
 publish = "site"
 command = "make build-docs"
+ignore = "git diff --quiet $CACHED_COMMIT_REF $COMMIT_REF ./docs"
 # available here https://github.com/netlify/build-image/blob/focal/included_software.md#languages
 environment = { PYTHON_VERSION = "3.8" }