Improved documentation

docs/troubleshooting.md

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+---
+layout: default
+title: Troubleshooting
+---
+
+# Troubleshooting
+Sometimes things go wrong and Part-DB shows an error message. This page should help you to solve the problem.
+
+## Error messages
+When a common, easy fixable error occurs (like a non up-to-date database), Part-DB will show you some short instructions on how to fix the problem. If you have a problem that is not listed here, please open an issue on GitHub.
+
+## General procedure
+If you encounter an error, try the following steps:
+* Clear cache of Part-DB with the console command: 
+```bash
+php bin/console cache:clear
+```
+* Check if the database needs an update (and perform it when needed) with the console command: 
+```bash
+php bin/console doctrine:migrations:migrate
+```
+
+If this does not help, please [open an issue on GitHub](https://github.com/Part-DB/Part-DB-symfony).
+
+## Error logs
+Detailed error logs can be found in the `var/log` directory.
+When Part-DB is installed directly, the errors are written to the `var/log/prod.log` file.
+
+When Part-DB is installed with Docker, the errors are written directly to the console output. 
+You can see the logs with the following command, when you are in the folder with the `docker-compose.yml` file
+```bash
+docker-compose logs -f
+```
+
+Please include the error logs in your issue on GitHub, if you open an issue.
+
+## Report Issue
+If an error occurs, or you found a bug, please [open an issue on GitHub](https://github.com/Part-DB/Part-DB-symfony).