feat: backups custom file names

[phaberest]

Documentation for the new Custom File Name Format feature for database and volume backups introduced in Dokploy/dokploy#3966.

Updated the backup related docs files to document:

• The new File Name Format field in backup configuration
• Available template variables ({timestamp}, {date}, {time}, {appName}, {volumeName}, {databaseType}, {uuid}, etc.)
• Example format patterns with expected outputs