chore(ui): change /devtest to /-/demo (#11019)

It has always been largely used for showcasing UI elements but that name didn't work too well for it.

Testing:
Some of existing tests depend on these pages, making it redundant to create extra tests.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/11019
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>

tailwind.config.js

@@ -28,8 +28,8 @@ export default {
   prefix: 'tw-',
   important: true, // the frameworks are mixed together, so tailwind needs to override other framework's styles
   content: [
-    isProduction && '!./templates/devtest/**/*',
-    isProduction && '!./web_src/js/standalone/devtest.js',
+    isProduction && '!./templates/demo/**/*',
+    isProduction && '!./web_src/js/standalone/demo.js',
     '!./templates/swagger/v1_json.tmpl',
     '!./templates/user/auth/oidc_wellknown.tmpl',
     './templates/**/*.tmpl',