Docs: Fix the way tabs are declared

Since python doesn't have a way to lock deps for a particlar project
by default, I didn't bother with it. This resulted in mkdocs updating at
some point, bringing a breaking change to how tabs are declared and
broken tabs on docs-develop.pleroma.social. I've learned my lesson
and locked deps with pipenv in pleroma/docs!5. This MR updates Pleroma
docs to use the new tab style, fortunately my editor did most of it.

Closes #2045

docs/configuration/static_dir.md

@@ -4,15 +4,19 @@ Static frontend files are shipped with pleroma. If you want to overwrite or upda
 
 You can find the location of the static directory in the [configuration](../cheatsheet/#instance).
 
-```elixir tab="OTP"
-config :pleroma, :instance,
-  static_dir: "/var/lib/pleroma/static/",
-```
+=== "OTP"
 
-```elixir tab="From Source"
-config :pleroma, :instance,
-  static_dir: "instance/static/",
-```
+    ```elixir
+    config :pleroma, :instance,
+      static_dir: "/var/lib/pleroma/static/"
+    ```
+
+=== "From Source"
+
+    ```elixir
+    config :pleroma, :instance,
+    static_dir: "instance/static/"
+    ```
 
 Alternatively, you can overwrite this value in your configuration to use a different static instance directory.