Update docs for prometheus and grafana metrics (#29823)
Update docs for prometheus and grafana metrics (#29823)

1. Renamed heading and Moved customize prometheus port section to bottom.
2. Add more instructions on how to download proper binaries.
3. Add screenshot for logging in as grafana admin user

Signed-off-by: e19aae48cb629e0f6c965d895071668ea9e4d7dd@anyscale.com
ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray,ray-project/ray