Deprecate GAP-Signature and add a warning on usage (#1103)

docs/docs/features/request_signatures.md

@@ -1,20 +0,0 @@
----
-id: request_signatures
-title: Request Signatures
----
-
-If `signature_key` is defined, proxied requests will be signed with the
-`GAP-Signature` header, which is a [Hash-based Message Authentication Code
-(HMAC)](https://en.wikipedia.org/wiki/Hash-based_message_authentication_code)
-of selected request information and the request body [see `SIGNATURE_HEADERS`
-in `oauthproxy.go`](https://github.com/oauth2-proxy/oauth2-proxy/blob/master/oauthproxy.go).
-
-`signature_key` must be of the form `algorithm:secretkey`, (ie: `signature_key = "sha1:secret0"`)
-
-For more information about HMAC request signature validation, read the
-following:
-
-- [Amazon Web Services: Signing and Authenticating REST
-  Requests](https://docs.aws.amazon.com/AmazonS3/latest/dev/RESTAuthentication.html)
-- [rc3.org: Using HMAC to authenticate Web service
-  requests](http://rc3.org/2011/12/02/using-hmac-to-authenticate-web-service-requests/)

docs/sidebars.js

@@ -18,7 +18,7 @@ module.exports = {
       type: 'category',
       label: 'Features',
       collapsed: false,
-      items: ['features/endpoints', 'features/request_signatures'],
+      items: ['features/endpoints'],
     },
     {
       type: 'category',