github: Add link to feedback use-case/testimonial (#4781)

**Which issue(s) this PR fixes**: 
Fixes #

**What this PR does / why we need it**: 

Add contact information to collect Fluentd's use-case

It help to collect Fluentd use-case widely.

**Docs Changes**:

N/A

**Release Note**: 

N/A

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

Author: kenhys

.github/ISSUE_TEMPLATE/config.yml

@@ -3,3 +3,7 @@ contact_links:
   - name: Ask a Question
     url: https://github.com/fluent/fluentd/discussions
     about: I have questions about Fluentd and plugins. Please ask and answer questions at https://github.com/fluent/fluentd/discussions
+feedback_testimonials:
+  - name: Feedback a Fluentd Use-Case/Testimonial
+    url: https://github.com/fluent/fluentd-website/issues/new?template=testimonials.yml
+    about: Feedback your Fluentd use-case/testimonial, How do you use Fluentd in your service?