fix(caddy): use language none for Caddyfile code blocks

Pygments has no Caddy lexer, so the nginx lexer was failing on Caddy's
brace syntax, causing Sphinx to emit warnings treated as errors in CI.

Signed-off-by: John Molakvoæ (skjnldsv) <skjnldsv@protonmail.com>

Author: skjnldsv

admin_manual/installation/caddy.rst

@@ -43,7 +43,7 @@ webroot of your Caddy installation. In this example it is
 ``/var/www/nextcloud`` and it is accessed via ``http(s)://cloud.example.com/``
 
 .. literalinclude:: Caddyfile.sample
-   :language: nginx
+   :language: none
 
 Nextcloud in a subdir of the Caddy webroot
 ------------------------------------------
@@ -60,7 +60,7 @@ The recommended approach is:
    or use ``uri strip_prefix /nextcloud``.
 3. Capture the rewritten URI before the PHP handler and pass it as ``REQUEST_URI``:
 
-.. code-block:: nginx
+.. code-block:: none
 
     handle_path /nextcloud/* {
         # … (place the route block contents here) …
@@ -90,7 +90,7 @@ If you're seeing meaningless messages in your logfile, for example ``client
 denied by server configuration: /var/www/data/htaccesstest.txt``, add this
 section to your Caddy configuration to suppress them:
 
-.. code-block:: nginx
+.. code-block:: none
 
         route {
           # …