Set up Renovate configuration with default and Docker-specific presets. Updated bot settings and added README.

README.md

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+# Renovate Configuration
+
+Base Renovate configuration for the self-hosted Gitea instance at `spenke-nordic.com`.
+
+## Usage
+
+Extend this configuration in your repository's `renovate.json`:
+
+```json
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "local>paulspenke/renovate-config:default"
+  ]
+}
+```
+
+## Presets
+
+- `default`: Base configuration with standard rules.
+- `docker`: Specific rules for Docker images.
+- `npm`: Specific rules for Node.js packages.
+- `github-actions`: Rules for GitHub/Gitea Actions.
+
+## Bot Configuration
+
+The bot is configured via `config.js` to run on the `gitea` platform.