docs: add `allow_public_signups` variable

docs/content/docs/configuration.md

@@ -20,6 +20,10 @@ the TOML file.
 * `log_level` (`OTTER_LOG_LEVEL`): how verbose the logging should be; one of
   `debug`, `info`, `warn` or `error`
     * Default: `warn`
+* `allow_public_signups` (`OTTER_ALLOW_PUBLIC_SIGNUPS`): whether public signups
+  are allowed. This enables the `/signup` route and adds a notice to sign up at
+  the bottom of the login page.
+    * Default: `false`
 
 ## Network (`net`)