remove POEditor export workflow and prune outdated translation keys

[solverat]

• Removed poeditor workflow and add some contribution guidelines if someone is willing to provide translations
• Normalized all admin.\*.yaml translation files: removed YAML document markers (---/...), decoded escaped unicode, collapsed multi-line values to single lines, wrapped all values in single quotes
• Sorted all translation files alphabetically by key
• Fixed typo in key name: csv_seperated_options => csv_separated_options and csv_seperated_options_info => csv_separated_options_info across all language files
• Removed obsolete key metainfo_copy_id from all language files
• Added 14 missing translation keys to admin.de.yaml
• Added 17 missing translation keys to admin_ext.de.yaml and removed 2 obsolete keys (database_administration, language_admin)