docs: Add error caddyfile directive (#143)

Docs for the new directive in https://github.com/caddyserver/caddy/pull/4034.

Also adding a bit in `handle_errors` mentioning that `reverse_proxy` doesn't trigger errors when a response has an "error" HTTP status; not sure how to word this clearly, cause `reverse_proxy` still can trigger errors if there's no upstream or whatever. We should probably add examples for that later, and augment this paragraph to mention `handle_response` once that's merged (https://github.com/caddyserver/caddy/pull/4021)

src/docs/markdown/caddyfile/directives/error.md

@@ -0,0 +1,46 @@
+---
+title: error (Caddyfile directive)
+---
+
+# error
+
+Triggers an error in the HTTP handler chain, with an optional message and recommended HTTP status code. 
+
+This handler does not write a response. Instead, it's meant to be paired with the [`handle_errors`](handle_errors) directive to invoke your custom error handling logic.
+
+
+## Syntax
+
+```caddy-d
+error [<matcher>] <status>|<message> [<status>] {
+    message <text>
+}
+```
+
+- **&lt;status&gt;** is the HTTP status code to write. Default is `500`.
+- **&lt;message&gt;** is the error message. Default is no error message.
+- **message** is an alternate way to provide an error message; convenient if it is multiple lines.
+
+To clarify, the first non-matcher argument can be either a 3-digit status code, or an error message string. If it is an error message, the next argument can be the status code.
+
+## Examples
+
+Trigger an error on certain request paths, and use [`handle_errors`](handle_errors) to write a response:
+
+```caddy
+example.com {
+	root * /srv
+
+	# Trigger errors for certain paths
+    error /private* "Unauthorized" 403
+	error /hidden* "Not found" 404
+
+    # Handle the error by serving an HTML page 
+    handle_errors {
+        rewrite * /{http.error.status_code}.html
+		file_server
+    }
+
+	file_server
+}
+```