[docs] global: update documentation

Author: bgallois

docs/user/assets/background.png

@@ -0,0 +1,13 @@
+---
+id: detection
+title: Detection tips
+sidebar_label: Detection
+---
+
+## Masking some regions
+
+It is sometimes necessary to mask specific regions to avoid reflection or artifacts and enhanced detection. We will show a straightforward manner with the movie [ZFL_004]( http://data.ljp.upmc.fr/datasets/TD2/#ZFL_004) from the two-dimensional dataset.
+
+1. Compute the background as usual. In this case, with will take the average.  ![background](assets/background.png)
+3. Construct a mask using [Gimp](https://www.gimp.org/) or [Inkscape](https://inkscape.org/) by taking the background image and masking irrelevant areas (in black or white, depending on the system).  ![background](assets/background_masked.png)
+5. Load the new background image in FastTrack and performs the tracking.  ![background](assets/multi_tracked.png)