Add the dark mode version of logo (#2063)

Author: Saransh-cpp

README.md

@@ -1,5 +1,6 @@
 <p align="center">
-<img width="400px" src="https://raw.githubusercontent.com/FluxML/fluxml.github.io/master/logo.png"/>
+    <img width="400px" src="https://raw.githubusercontent.com/FluxML/Flux.jl/master/docs/src/assets/logo.png#gh-light-mode-only"/>
+    <img width="400px" src="https://raw.githubusercontent.com/FluxML/Flux.jl/master/docs/src/assets/logo-dark.png#gh-dark-mode-only"/>
 </p>
 
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docs/make.jl

@@ -42,6 +42,7 @@ makedocs(
          "Flux's Ecosystem" => "ecosystem.md",
     ],
     format = Documenter.HTML(
+        sidebar_sitename = false,
         analytics = "UA-36890222-9",
         assets = ["assets/flux.css"],
         prettyurls = get(ENV, "CI", nothing) == "true"