add german translation, update docs (#931)

- add german translation (following the changes in
https://github.com/maplibre/maputnik/pull/929)
- using an alphabecial order for the languages
- update the documentation with a step-by-step guide
- add myself as a helping person for german
- move the helping-users list to the implementation of new features
- update CHANGELOG.md
- add a link to the l18n README in the root README


![image](https://github.com/user-attachments/assets/2834f2c5-2cb7-4848-a6bf-24149be979ae)

i18next-parser.config.ts

@@ -1,6 +1,6 @@
 export default {
   output: 'src/locales/$LOCALE/$NAMESPACE.json',
-  locales: [ 'fr', 'ja', 'he','zh' ],
+  locales: [ 'de', 'fr', 'he', 'ja', 'zh' ],
 
   // Because some keys are dynamically generated, i18next-parser can't detect them.
   // We add these keys manually, so we don't want to remove them.