Fix mdox link validation CI failure for medium.com 403 responses (#819)

* Initial plan

* Fix lint CI: add mdox validate config to ignore medium.com 403 errors

Co-authored-by: johanbrandhorst <6604151+johanbrandhorst@users.noreply.github.com>
Agent-Logs-Url: https://github.com/grpc-ecosystem/go-grpc-middleware/sessions/b880671a-fa03-46cd-862e-7e07c6e894d8

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: johanbrandhorst <6604151+johanbrandhorst@users.noreply.github.com>

Author: Copilot

.mdox.validate.yaml

@@ -0,0 +1,5 @@
+version: 1
+
+validators:
+  - regex: 'medium\.com'
+    type: 'ignore'

Makefile

@@ -7,6 +7,7 @@ MODULES          ?= $(PROVIDER_MODULES) $(PWD) $(PWD)/examples
 GO_FILES_TO_FMT  ?= $(shell find . -path -prune -o -name '*.go' -print)
 
 GOBIN             ?= $(firstword $(subst :, ,${GOPATH}))/bin
+MDOX_VALIDATE_CONFIG ?= .mdox.validate.yaml
 
 TMP_GOPATH        ?= /tmp/gopath