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/administration/CLI_tasks/robots_txt.md

@@ -8,10 +8,14 @@ The `robots.txt` that ships by default is permissive. It allows well-behaved sea
 
 If you want to generate a restrictive `robots.txt`, you can run the following mix task. The generated `robots.txt` will be written in your instance [static directory](../../../configuration/static_dir/).
 
-```elixir tab="OTP"
-./bin/pleroma_ctl robots_txt disallow_all
-```
+=== "OTP"
 
-```elixir tab="From Source"
-mix pleroma.robots_txt disallow_all
-```
+    ```sh
+    ./bin/pleroma_ctl robots_txt disallow_all
+    ```
+
+=== "From Source"
+
+    ```sh
+    mix pleroma.robots_txt disallow_all
+    ```