docs: router HTTP response status codes (#4733)

Document router response status codes in [new errors
page](https://deploy-preview-4733--apollo-router-docs.netlify.app/errors)

Resolves #4740 and
https://apollographql.atlassian.net/browse/DOC-34

---------

Co-authored-by: Jesse Rosenberger <[email protected]>
Co-authored-by: Maria Elisabeth Schreiber <[email protected]>

docs/source/config.json

@@ -33,6 +33,7 @@
         ]
       },
       "Debugging": {
+        "Errors": "/errors",
         "Telemetry": "/configuration/telemetry/overview",
         "Subgraph error inclusion": "/configuration/subgraph-error-inclusion"
       },

docs/source/errors.mdx

@@ -0,0 +1,61 @@
+---
+title: Errors
+subtitle: Error and status codes returned by the Apollo Router
+description: Error codes and HTTP response status codes returned by the Apollo Router
+---
+
+Learn about error codes and HTTP response status codes returned by the Apollo Router.
+
+## Status codes
+
+<PropertyList kind="errCodes">
+<Property name="400" short="Bad request">
+
+A request failed GraphQL validation or failed to be parsed.
+
+</Property>
+<Property name="401" short="Unauthorized">
+
+Requests may receive this response in two cases:
+
+- For a client request that requires authentication, the client's JWT failed verification.
+- For a non-client subscription endpoint calling a subscription callback URL, the router couldn't find a matching subscription identifier between its registered subscriptions and a subscription event.
+
+</Property>
+<Property name="405" short="Method not allowed">
+
+A request used an unallowed HTTP method.
+
+<Note> 
+
+Both mutations and subscriptions must use POST.
+
+</Note>
+
+</Property>
+<Property name="406" short="Not acceptable">
+
+A request's HTTP `Accept` header didn't contain any of the router's supported mime-types: 
+- `application/json`
+- `application/graphql-response+json`
+- `multipart/mixed;deferSpec=20220824`
+- `multipart/mixed;subscriptionSpec=1.0`.
+
+</Property>
+<Property name="429" short="Too many requests">
+
+Request traffic exceeded configured rate limits. See [client side traffic shaping](/configuration/traffic-shaping/#client-side-traffic-shaping).
+
+</Property>
+<Property name="500" short="Internal server error">
+
+The router encountered an unexpected issue. [Report](https://github.com/apollographql/router/issues/new?assignees=&labels=raised+by+user&projects=&template=bug_report.md&title=) this possible bug to the router team.
+
+</Property>
+</PropertyList>
+
+<Note>
+
+You can create Rhai scripts that throw custom status codes. See [Terminating client requests](/customizations/rhai-api#terminating-client-requests) to learn more.
+
+</Note>