Add new sample simple_sdf (#2835)

Add a new sample, `simple_sdf`. This is similar to `simple_shader`, but
it focuses on using Signed Distance Functions within the fragment
shader.

Relates to issue
[#183043](https://github.com/flutter/flutter/issues/183043)

## Pre-launch Checklist

- [x] I read the [Flutter Style Guide] _recently_, and have followed its
advice.
- [x] I signed the [CLA].
- [x] I read the [Contributors Guide].
- [ ] I have added sample code updates to the [changelog].
- [x] I updated/added relevant documentation (doc comments with `///`).


If you need help, consider asking for advice on the #hackers-devrel
channel on [Discord].

<!-- Links -->
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md
[CLA]: https://cla.developers.google.com/
[Discord]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Chat.md
[Contributors Guide]:
https://github.com/flutter/samples/blob/main/CONTRIBUTING.md
[changelog]: ../CHANGELOG.md

.github/dependabot.yaml

@@ -281,6 +281,12 @@ updates:
       interval: "daily"
     labels:
       - "autosubmit"
+  - package-ecosystem: "pub"
+    directory: "simple_sdf/"
+    schedule:
+      interval: "daily"
+    labels:
+      - "autosubmit"
   - package-ecosystem: "pub"
     directory: "simple_shader/"
     schedule: