Provide reusable presets for the most common dependency types in Gitea projects (#97)

Renovate by default doesn't group dependencies. This is to achieve fine-grained changes. However, it can also be the root cause for maintainers getting flooded with notifications.
With regards to Renovate [noise reduction](https://docs.renovatebot.com/noise-reduction/) and related to #93, one of the most powerful ways is grouping related dependencies into a single Pull Request.

This introduces the following reusable presets:

- Golang dependencies
- NPM dependencies
- Workflow/Actions dependencies
- Security fixes (depends on an experimental feature: https://github.com/renovatebot/renovate/discussions/20542)

Combining those presets with proper Pull Request scheduling, they can help finding a good workflow to keep up-to-date.

---

A heads-up about possible downsides of dependency grouping. If a group contains an update that is not possible, one has to either:
- manually adjust the Pull Request to not include that package update
- manually adjust the repository `renovate.json5` to ignore a specific package version

Reviewed-on: https://gitea.com/gitea/renovate-config/pulls/97
Reviewed-by: Jason Song <i@wolfogre.com>
Co-authored-by: justusbunsi <sk.bunsenbrenner@gmail.com>
Co-committed-by: justusbunsi <sk.bunsenbrenner@gmail.com>

workflow-deps.json

@@ -0,0 +1,12 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "github-actions": {
+    "enabled": true
+  },
+  "packageRules": [
+    {
+      "groupName": "workflow dependencies",
+      "matchManagers": ["github-actions"]
+    }
+  ]
+}