Document an end-to-end SciTokens XRootD Demo

[jbasney]

Create a "how to" doc that shows:

1. How to run an XRootD server using https://github.com/scitokens/docker-xrootd-scitokens
2. How to run a SciToken server using https://github.com/scitokens/docker-scitokens-java
3. How to use https://indigo-dc.gitbook.io/oidc-agent/ to get a token from the SciToken server and use that token to access the XRootD server.