chore(metadata): add plugin metadata + improve contributor guidelines

.github/pull_request_template.md

@@ -72,6 +72,7 @@ Thank you for your contribution. ❤️  -->
 - [ ] Use `"{{ secret('YOUR_SECRET') }}"` in the examples for sensible infos such as an API KEY.
 - [ ] If you are fetching data (one, many or too many), you must add a `Property<FetchType> fetchType` to be able to use `FETCH_ONE`, `FETCH` and even `STORE` to store big amount of data in the internal storage.
 - [ ] Align the `"""` to close examples blocks with the flow id.
+- [ ] Update the existing `index.yaml` for the main plugin, and for each new subpackage add a metadata file named exactly after the subpackage (e.g. `s3.yaml` for `io.kestra.plugin.aws.s3`) under `src/main/resources/metadata/`, following the same schema.
 
 🧪 **Tests**
 - [ ] Unit Tests added or updated to cover the change (using the `RunContext` to actually run tasks).
@@ -83,3 +84,5 @@ Thank you for your contribution. ❤️  -->
 - [ ] Do not send back as outputs the same infos you already have in your properties.
 - [ ] If you do not have any output use `VoidOutput`.
 - [ ] Do not output twice the same infos (eg: a status code, an error code saying the same thing...).
+
+

src/main/resources/metadata/index.yaml

@@ -0,0 +1,8 @@
+group: io.kestra.plugin.template
+name: "template"
+title: "Template"
+description: "Plugin template for Kestra"
+body: ""
+videos: []
+createdBy: "Kestra Core Team"
+managedBy: "Kestra Core Team"