feat: complete PR template frontmatter fields (#8)

## Description

Adds all recommended frontmatter fields to the Gitea pull request template.
This includes `about`, `ref`, `assignees`, and `labels` fields in addition to the required `name` and `title` fields.
These fields provide better template documentation and default PR settings.

## Benefits

- The PR template now includes a description (`about`) explaining its purpose.
- Default target branch is explicitly set to `main` via the `ref` field.
- Template follows Gitea best practices with all available frontmatter fields.
- Provides a complete, well-documented template structure for contributors.

## Checklist

- [x] Code follows conventional commit format.
- [x] Branch follows naming convention (`<type>/<description>`).
- [x] Tests pass (if applicable).
- [x] Documentation updated (if applicable).

Reviewed-on: #8
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

.gitea/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,10 @@
 ---
 name: "Default Template"
+about: "The default template for `lambda`."
 title: "<type>: <description>"
+ref: "main"
+assignees: []
+labels: []
 ---
 
 ## Description