[demos] Add directory from which to serve demos.skia.org

To run the demos locally, cd in to the directory and run
  make local

Then navigate to, for example, http://localhost:8123/demos/hello_world/

Change-Id: I5954989053d4602cd558c646e19106998dc00604
Bug: skia:10170
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/298397
Reviewed-by: Weston Tracey <westont@google.com>
2 files changed