Mastering XDS is the key step of replacing envoy in waypoint with orion.

Now we have some example code (https://github.com/kmesh-net/orion/tree/main/orion-xds/examples) on how to use xds lib of orion but lack of doc.

Need a README of how to run these examples and different usage of code.

*** NOTE: not only doc but also make sure the code could run smoothly ***