feat: Add option for a noindex meta tag on the HTML index page

Adds a new config option `index_page_allow_indexing` which defaults to false.

Fixes: !1527

conduwuit-example.toml

@@ -1799,6 +1799,11 @@
 #
 #config_reload_signal = true
 
+# Allow or disallow search engine crawling by adding
+# `<meta name="robots" content="noindex" />` to the index page.
+#
+#index_page_allow_indexing = false
+
 [global.tls]
 
 # Path to a valid TLS certificate file.