feat: Remove NGINX Service Mesh documentation (#1399)

This commit removes the documentation for NGINX Service Mesh, which went
to end of sale on July 1st 2023 and was formally archived on March 14th
2024. It removes the entire content folder for the project, as well as
static example files for Grafana dashboards.

Author: ADubhlaoich

.github/workflows/linkchecker.yml

@@ -56,7 +56,6 @@ jobs:
           - waf
           - nginx-ingress-controller
           - nginxaas/azure
-          - nginx-service-mesh
           - nginx-amplify
           - nginx-controller
           - nginx-waf