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/instance.md

@@ -3,13 +3,17 @@
 {! backend/administration/CLI_tasks/general_cli_task_info.include !}
 
 ## Generate a new configuration file
-```sh tab="OTP"
- ./bin/pleroma_ctl instance gen [option ...]
-```
+=== "OTP"
 
-```sh tab="From Source"
-mix pleroma.instance gen [option ...]
-```
+    ```sh
+     ./bin/pleroma_ctl instance gen [option ...]
+    ```
+
+=== "From Source"
+
+    ```sh
+    mix pleroma.instance gen [option ...]
+    ```
 
 
 If any of the options are left unspecified, you will be prompted interactively.