gitea png to logo (#13974)

* gitea png to logo.svg

* gitea safari svg to logo

* minify svgs

* Update english docs

* Update missing section on customizing logo

snap/snapcraft.yaml

@@ -6,7 +6,7 @@ description: |
   an independent binary distribution across ALL platforms that Go supports,
   including Linux, Mac OS X, Windows and ARM.
 
-icon: public/img/gitea-lg.png
+icon: public/img/logo-lg.png
 confinement: strict
 base: core18
 adopt-info: gitea