Add OCR preferences to user docs (#637)

Author: ZiadAbdElFatah

en/advanced/OCR.md

@@ -15,12 +15,49 @@ OCRmyPDF must be installed on your system to use this feature.
 1. Open JabRef and select the entry with the scanned PDF you want to OCR.
 2. Open the General Tab of the Entry Editor.
 3. Right-click on the File and select "Perform OCR and embed text into new PDF file".
-    ![perform-ocr.png](perform-ocr.png)
+   ![perform-ocr.png](perform-ocr.png)
 
 * After performing OCR, JabRef creates a new PDF file with the OCR text embedded, and it will be linked to all the entries that have the old file linked to them. The original scanned PDF will remain unchanged.
 
-    ![original-and-ocred-files.png](original-and-ocred-files.png)
+  ![original-and-ocred-files.png](original-and-ocred-files.png)
 
 * Now you can select and search text in the new PDF file.
 
-    ![comparison-between-original-and-ocred-file.png](comparison-between-original-and-ocred-file.png)
+  ![comparison-between-original-and-ocred-file.png](comparison-between-original-and-ocred-file.png)
+
+## OCR Preferences
+
+* The OCR preferences can be accessed via **File → Preferences → OCR**.
+
+  ![ocr-preferences.png](ocr-preferences.png)
+
+### Engine Path
+
+{% hint style="warning" %}
+Performing OCR will fail if wrong engine path is provided, make sure that the correct path is provided.
+{% endhint %}
+
+* JabRef needs to know the location of the OCRmyPDF executable to run OCR. By default, JabRef assumes `ocrmypdf` is available on your system PATH, which is the case for most standard installations.
+
+* If OCRmyPDF is installed in a non-standard location, or if it needs to be invoked through Python, you can configure the path manually in this preference tab.
+
+* There are three ways to set the engine path:
+
+1. **Type the path manually**: Enter the path directly into the text field. This can be a bare command name (e.g. `ocrmypdf` or `python -m ocrmypdf`) if it is available on your system PATH, or a full absolute path to the executable (e.g. `/home/user/.local/bin/ocrmypdf`).
+
+   ![text-field-for-engine-path.png](text-field-for-engine-path.png)
+
+2. **Browse**: Click the folder icon to open a file chooser and navigate to the OCRmyPDF executable on your system.
+
+   ![browse-engine-path-button.png](browse-engine-path-button.png)
+
+3. **Auto-detect**: Click the magnifier icon to have JabRef automatically search for a working OCRmyPDF installation. JabRef will try the following commands in order and use the first one that works:
+
+   ![auto-detect-engine-path-button.png](auto-detect-engine-path-button.png)
+
+   1. `ocrmypdf`
+   2. `python -m ocrmypdf`
+   3. `py -m ocrmypdf`
+   4. `python3 -m ocrmypdf`
+
+If none of these succeed, the path field will remain unchanged and you will need to set the path manually.