feat(sidebar): add support for external avatar (#53)

add [params.sidebar.avatar] section in config.toml, with 2 fields: local and src

Migration guide:
 - Copy the old value of `params.sidebar.avatar` to `param.sidebar.avatar.src`
 - Set params.sidebar.avatar.local = true

Co-authored-by: Jimmy Cai <jimmehcai@gmail.com>

exampleSite/config.toml

@@ -21,8 +21,10 @@ DefaultContentLanguage = "en"                   # Theme i18n support
         
     [params.sidebar]
         emoji = "🍥"
-        avatar = "img/avatar.png"
         subtitle = "Lorem ipsum dolor sit amet, consectetur adipiscing elit."
+        [params.sidebar.avatar]
+            local = true
+            src = "img/avatar.png"
         
     [params.article]
         math = false