FEAT: Ability to set custom birdseye icon and birdseye docs (#2979)

* Show custom.png for birdseye icon if available

* Don't look for config value

* Add birdseye docs

docs/docs/configuration/birdseye.md

@@ -0,0 +1,14 @@
+# Birdseye
+
+Birdseye allows a heads-up view of your cameras to see what is going on around your property / space without having to watch all cameras that may have nothing happening. Birdseye allows specific modes that intelligently show and disappear based on what you care about. 
+
+### Birdseye Modes
+
+Birdseye offers different modes to customize which cameras show under which circumstances.
+ - **continuous:** All cameras are always included
+ - **motion:** Cameras that have detected motion within the last 30 seconds are included
+ - **objects:** Cameras that have tracked an active object within the last 30 seconds are included
+
+### Custom Birdseye Icon
+
+A custom icon can be added to the birdseye background by provided a file `custom.png` inside of the Frigate `media` folder. The file must be a png with the icon as transparent, any non-transparent pixels will be white when displayed in the birdseye view. 

docs/sidebars.js

@@ -22,6 +22,7 @@ module.exports = {
       'configuration/objects',
       'configuration/rtmp',
       'configuration/zones',
+      'configuration/birdseye',
       'configuration/advanced',
       'configuration/hardware_acceleration',
       'configuration/nvdec',