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/email.md

@@ -4,21 +4,29 @@
 
 ## Send test email (instance email by default)
 
-```sh tab="OTP"
- ./bin/pleroma_ctl email test [--to <destination email address>]
-```
+=== "OTP"
 
-```sh tab="From Source"
-mix pleroma.email test [--to <destination email address>]
-```
+    ```sh
+     ./bin/pleroma_ctl email test [--to <destination email address>]
+    ```
+
+=== "From Source"
+
+    ```sh
+    mix pleroma.email test [--to <destination email address>]
+    ```
 
 
 Example: 
 
-```sh tab="OTP"
-./bin/pleroma_ctl email test --to root@example.org
-```
+=== "OTP"
 
-```sh tab="From Source"
-mix pleroma.email test --to root@example.org
-```
+    ```sh
+    ./bin/pleroma_ctl email test --to root@example.org
+    ```
+
+=== "From Source"
+
+    ```sh
+    mix pleroma.email test --to root@example.org
+    ```