Create docs site favicon

Favicon created by running
```
$ convert logo.png -scale 128 favicon.ico
```
using ImageMagick in docs/src/assets

Author: abhro

docs/make.jl

@@ -27,7 +27,7 @@ makedocs(;
     modules=[ChainRulesCore],
     format=Documenter.HTML(;
         prettyurls=false,
-        assets=[indigo],
+        assets=[indigo, "assets/favicon.ico"],
         mathengine=MathJax3(
             Dict(
                 :tex => Dict(