.github/release.yml: Added "Improvements" category and updated docs (apache#1015)

Author: NightOwl888

.github/release.yml

@@ -17,6 +17,9 @@
     - title: 🚀 Performance Improvements
       labels:
         - notes:performance-improvement
+    - title: 🏆 Improvements
+      labels:
+        - notes:improvement
     - title: 📄 Website and API Documentation
       labels:
         - notes:website-or-documentation

websites/site/contributing/make-release.md

@@ -167,6 +167,7 @@ We have partially automated the release notes through GitHub Releases. They are
 | notes:new-feature              | Categorizes the PR under "New Features"                  |
 | notes:bug-fix                  | Categorizes the PR under "Bug Fixes"                     |
 | notes:performance-improvement  | Categorizes the PR under "Performance Improvements"      |
+| notes:improvement              | Categorizes the PR under "Improvements"                  |
 | notes:website-or-documentation | Categorizes the PR under "Website and API Documentation" |
 | \<none of the above\>          | Categorizes the PR under "Other Changes"                 |