Restructure documentation

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

docs/chapters/ocr_backend.txt

@@ -0,0 +1,26 @@
+===========
+OCR backend
+===========
+
+Mayan EDMS ships an OCR backend that uses the FLOSS engine Tesseract
+(https://github.com/tesseract-ocr/tesseract/), but it can
+use other engines. To support other engines create a wrapper that subclasses the
+``OCRBackendBase`` class defined in mayan/apps/ocr/classes. This subclass should
+expose the ``execute`` method. For an example of how the Tesseract backend
+is implemented take a look at the file ``mayan/apps/ocr/backends/tesseract.py``
+
+Once you create you own backend, in your local.py settings add the option
+OCR_BACKEND and point it to your new OCR backend class path.
+
+The default value of OCR_BACKEND is ``"ocr.backends.tesseract.Tesseract"``
+
+To add support to OCR more languages when using Tesseract, install the
+corresponding language file. If using a Debian based OS, this command will
+display the available language files:
+
+    apt-cache search tesseract-ocr
+
+If using the Docker image, pass the environment variable MAYAN_APT_INSTALLS
+with the corresponding Tesseract language option. Example::
+
+    -e MAYAN_APT_INSTALLS='tesseract-ocr-deu'