feat: Add Template Grafana Dashboard for ModelMesh Metrics (kserve#346)

#### Motivation
To port a previously-built Grafana dashboard developed for displaying various prometheus metrics for ModelMesh and build upon it to include metrics at the deployment/runtime-view

#### Modifications
- Added `config/dashboard/` directory to host grafana dashboard JSON
- Added `servicemonitor.yaml` to `config/prometheus/`
- Added and modified `ModelMeshMetricsDashboard.json` to work out-of-the-box
  - Added queries for deployment views to filter by serving runtime
  - Created sections for global-view and deployment-view visualizations
  - Created drop-down variables for filtering between one or many pre-existing runtimes with option to filter by custom serving runtime
- Updated `monitoring.md` doc to include guide for setting up Prometheus/Grafana for monitoring; mainly using instructions from [modelmesh-performance](https://github.com/kserve/modelmesh-performance/tree/main/docs/monitoring)

#### Result
- A functional, ready-to-use dashboard JSON
- General documentation outlining out to set up monitoring and use the JSON
- Closes kserve#335

Signed-off-by: Rafael Vasquez <raf.vasquez@ibm.com>
Signed-off-by: Luis Delossantos <luisgd@ibm.com>